Top 10 Common Mistakes In Fish Keeping

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My New Aquarium

My new Aquarium in which I made many of the common mistakes.

Hello eveybody. As a new fish keeper myself I have asked countless questions and have read countless articles about it and have come across many of the same things over and over again. I am going to put the ten most talked about topics in one place and hope it helps someone not make the same mistakes I have made.

1. One of the most common mistakes in fish keeping is not cycling your tank. You go out get a fish, plop it in some water and call it a day. This common mistake can lead to fish illness and even fish death. Cycling is a process in which good bacteria grow to deal with ammonia, and nitrite in the water. Ammonia comes from fish food and fish waste. You will get it no matter what you do. Cycling your tank takes time. This time can vary based on the size of your tank, whether you have fish already, if you use live bacteria to “jump start” your tank and what you “Feed” your tank with. I am currently cycling my tank with fish and have come to realize how difficult this process is. I would recommend to anyone reading this to do a fishless cycle. There are many good articles detailing a fishless cycle already: and If you just HAVE to have that perfect fish (and I strongly recommend you don’t do this) then you are in for an uphill battle. Daily and I mean that daily partial water changes and daily testing for ammonia, nitrite and nitrate are in store for you.

2. The next mistake that I have seen a lot of is overstocking. This means too many fish for too small a space. Lots of people find a million fish they like and want to keep but only have a five gallon tank. Fish need space to swim in to be happy and comfortable. The general rule of thumb is one inch of grown up fish ((They get bigger!)) to 1 gallon of water. Now this is only a loose rule as some fish are much more messy ((Like the common goldfish)) and they need more room. Some fish get aggressive in small spaces and will injure other fish if your tank is too small. Others like to live in large groups of their own kind and will be very unhappy alone or in pairs.

3. Number two leads straight to number three. The fish bowl. We’ve all done it. You win a fish or get one gifted to you and the only spot you have available is the fish bowl or that vase your mother-in-law gave you seven years ago. These items are TOO SMALL for actual fish! The water quickly becomes polluted and as the fish grows they lose the very small bit of swimming room they may have had when they were juveniles. Please get AT LEAST a five gallon tank. Remember bigger in this case is always better!

4. Another common mistakes is equipment. ALL FISH need filtration. Even that little Betta you got in the cup. A lot of beneficial bacteria live in the filter and it adds oxygen to the water which fish need to breathe. Filtration, be a little gentle sponge filter or a turbo boosted “Home Improvement” style filter is needed to get some of the fish waste and gunk out of your tank. A little side note here, DON’T clean your filters under tap water! This water holds chemicals that kill your bacteria. Swish them gently in a bucket of used tank water. When replacing filters ((Also note: you don’t need to do this often, white filters are actually bad as you are replacing all your bacteria with no bacteria)) place the old one in front or behind the new one or cut it up and put it in with your new filter so your tank doesn’t lose all your beneficial bacteria.

5. Now for some more chemistry: Ph. Check your Ph levels once a week and try to avoid using aquarium products like Ph up or Ph down, these are TEMPORARY solutions at best. Most fish can adjust to any stable Ph value, its rapid fluctuations that are truly harmful. If you have low Ph adding crushed coral or shells, limestone works well too, to your tank or to the filter is usually a good idea. If you have high Ph and want to lower it, use some peat in a mesh bag ((Warning this MAY discolour your water)) or a piece of wood can help too. Just remember most fishes will be okay with a strange Ph. If you are changing your PH don’t do it all at once, lower or raise it gradually so your fish are not shocked.

6. This one is an important one.. Most people go to the pet store, see a pretty fish or two, buy it and mesh the fish in with ones they already have. I STRONGLY suggest you research the needs of your fish. They may be pretty and add that perfect splash of colour to your tank but you may wake up to a missing fish or even worse a dead one. All fish live in certain parts of the world for a reason, they have adapted over centuries to live there. Temperature, behaviour, survival instinct, chemical balance all these things are a need to know BEFORE you buy your fish and put them in the tank. Goldfish for example are COLD WATER fishes and can’t survive well in heated water with tropicals. Male bettas are fighting fish, they will nip the fins of other fish when they are elaborate and they generally need a soft gentle filter as their fins slow them down. Angel fish are predatory, they will eat smaller fish and since they are taller than they are long, they prefer a deep tank. All this is important stuff to know.

7. Feeding your fish. Feeding your fish may be more complicated than you know. Lots of people buy flake food and leave it at that. Fish are like people they need variety in their diet. Live food, plant material and protein plus vitamins and minerals are all important. KNOW what your fish eats: is it an herbivore? ((eats nothing but plant material)) Is it a carnivore? ((Eats nothing but meat with very little plants)) Or is it an omnivore? ((Eats both meat and plants)). There is a remarkable range of foods available for fish these days, Live, flakes, pellets, wafers, frozen, freeze dried and “people food” like peas and lettuce. Remember if you feed your fish frozen food, thaw it out first.

8. Seven leads to eight.. Overfeeding. Again a very common mistake and it may take you a couple days or even weeks ((Weeks in my case, I have mollies which are little beggars. Even if they are not hungry they will “Beg” for food then spit it out when I give in to them)) to figure out how much your fishes eat and how much gets wasted. Wasting the food isn’t the problem here, its your money, its that decaying food adds to your bio-load ((what the bacteria can process)). Food decaying on the bottom of your tank can produce excess ammonia and overwhelm your beneficial bacteria.

9. This is a common mistake that I see happen more often than not. Over cleaning. You think you are being a good owner by keeping the tank sparkly clean for your fishes but really what you are doing is scrubbing or sucking up all your bacteria. Filters are supposed to be brown! Never use soaps or detergents on your tank or tank items ((like decorations or gravel)). They carry harmful substances for your fish. You should never have to “break down” a tank. Most literature says to do this once a year, but if you keep up with weekly maintenance you shouldn’t have to dismantle everything and start over. Get a gravel vacuum and use it to suck out any waste hiding in the substrate ((the stuff on the bottom of your tank, gravel, sand, peat, marbles)). Gently swish the filter media (the pad thingy) in OLD TANK WATER, not tap water. You can keep on top of algae but getting a fish that likes to eat it ((Like mollies or plecos.. make sure you have room for them and do your research!)) or by getting a gentle scrub brush or magnetized glass cleaner. Reduce the amount of light your tank gets (Never place your tank in or by a window!) a day and do your partial water changes once or twice a week to get rid of built up nitrAtes. Live plants can help with nitrAte excess but they are not completely needed if you keep up with water changes.

10. Under cleaning. Having a tank comes with responsibility. You are responsible for keeping the fishes environment clean and free of toxins. Do WEEKLY partial (25-50%) water changes and use a gravel vacuum to get left over food. ((Never clean all the gravel or all of the filter at once!)) Test your water weekly for ammonia, nitrite and nitrAte. Ammonia should be zero, nitrite should be zero and NitrAte should be less than 40 ppm. Water from the tap NEEDS to be conditioned to get rid of chlorine and chloramines and heavy metals. Make sure the temperature of the replacement water is the as close to the water in the tank as you can get it.

So that is it, the 10 most common mistakes. There are more to make I am sure, I didn’t cover anything about baby fishes and fish stress or sickness. I didn’t cover the death of a fish and what to do if you have to put a fish out of its misery. Personally I think baby fishes (fry) should come with a how-to-do guide themselves. The best I can say is keep reading.. You’ll never know when you come across the perfect solution that meets your specific needs. This site and many others cover lots of the basics for new beginners! Talk to other people that keep fishes, something they do might help you out. Be wary of generic pet stores as most often then not they are just doing their jobs and trying to sell you stuff. Always err on the side of caution when dealing with chemicals.

So... what do you think? Please leave me a comment or give me a


  • KasiaJB: Thank you Wythori! Fantastic read and so many useful information and tips for everyone - beginners and those more advanced aquarists.
  • johnarthur: You’ve covered some very important issues. Certainly any new aquarist will benefit from reading your blog, and we old timers can learn a thing or two from it too. I really wish that information had been available back in the 50s.
  • Chazscalare: Very good post.
    A lot of good info for newbies and others and for people who want to know as much as they can so we can try and learn and keep a healthy aquarium environment for our fish
    Really enjoyed reading that
  • Margaret: Oh, thank you, that’s just brilliant! I’ve started looking after my fish better (it was the sparkling clean one.....!!)and I’m growing the bacteria :D
  • lynnmarie0123: AWESOME AWESOME information. stuff i would have never considered. thank you for this extremel helpful guide.
  • cross: very informative
  • Naveen: An excellent article. Puts all the things I needed to know in ONE place. Brilliant THANKS
  • tmjohnson: Thank you! Very helpful.
  • TomB: Thank you for an informative site. I grew up with freshwater aquariums and worked in pet stores but it has been a long time since I have done that. Your explanations brought back so many wonderful memories of helping customers understand the relationship between fish and their environment. I miss those days. :(
  • Philter: They should make you read this before you buy an aquarium. #11f for me would be a quarantine tank.
  • RainbowBreeze: Awesome!! Great top 10 list for anyone looking into or still new (including me) at figuring all of these details out! :)
  • PrincessKimmy: Thank you.
  • sue: Ihave just set up a new tank three months ago and have made a few mistakes on reading your 10 tips it has helped me a lot thank you.
  • Lindsey: Ok u change ur filters once a month whoever said don’t wash it in tap is full of **** i raise Angeles and mine are huge i also only do a tank change once a month about 10 gallons in a 35 gallon tank i put tap to replace and when i filled the breeder tank i put tap in it with a bag of dirty water and cycled it for two weeks have never had a prob make sure u float the fish and buy cheep live plants these people who said all the above probably own a pet shop and want u to spend 100s on ph testers.....i have blue angels scattered scale angels platinum angels plecos tetras a beta and dynos.....keep angels between 80-85 degrees they are happy and the goal to keep angels happy and alive is for every half dollar size fish have at least a gallon of water and hex or tall tanks are best and u will have better luck if u have a larger filter than needed i have 35 galllon will a 55 gal double above ground filter they thrive on red worms and flake food... and ur angel needs to b a year b4 its gonna breed...and for every 6 fish u buy one pair wil pair up.
  • johnarthur: Most of the beneficial bacteria associated with the nitrogen cycle live in the filter elements and substrate. If you rinse a filter element in tap water, it will rinse out most of the beneficial bacteria. That may have very little effect if the aquarium has multiple filter elements and a good substrate.
  • alex6163: you are very smart,thank you for helping the struggle of stupid five year olds with 1 gram tanks (ITS AN EXXAGERATION)the first tank i got when i was a beginner was a 1 \gallon filtered tank with a light hood,plus a crowntail betta ,anyway,good story!
  • Joe Aro: Excellent information. I received my first tropicals nearly 60 years ago and have had aquaria off and on over the years. You are correct that one must put in lots of care to have a healthy and enjoyable environment for our fish friends. Sad too many treat aquarium fish as disposable items rather than the wonder they are.
  • youngjsampson: This is a good post. All things I could see to be major problems in my new aquarium after reading much online. Seems better to take you time and make sure things are in line before adding too much to the already tricky equation.
  • diane04068: Fantastic all is covered!
  • athena: This is very helpful for people who are new to fish keeping!
  • steeler07: Very informative!!! Opened my eyes to a couple newbie mistakes I was making!!! Thank you!!!
  • airdrieaquarist: All the information you need in one post. well done!
  • Chia4855: Wow, that was great. But I want to know how to set up a new aquarium for my goldfish. How would you recommend to do that? I have a live bamboo plant too but it needs light so I’m not sure how to set that up with my goldfish. He is currently recovering from ammonia burns so I wanted to get him a better tank.
    However, I do not have a gravel vacuum or a filter and I would rather not get one. How can I get my tank to have a nitrogen cycle before putting him in? How would I feed the bacteria?
    Thank you!
  • EnviousBakemono: Thanks I learned a lot and will use this information in the future :)
  • Ajay: Great information for the beginner as well as for the enthusiast!!
  • Ky: I’m relieved to see that I know of all these common mistakes (now)... although when I first started I was guilty of almost all of them. I guess this means I’m well on my way to becoming an educated aquarist now and DANG that feels good! I can’t thank this community enough for helping people like me. Your wisdom is Very appreciated!
  • fishygirl: Great blog. Easy to understand for the inexperienced and highlights the really important things for fishy health. I am currently struggling as had to re house my fish in a new tank after old one sprang an irreparable leak. So doing lots of water changes etc. No one has died yet but Colin the Cory even more of a loner than before and hides all the time. Worry about him - note to self - get life !!
  • Starshine: fishygirl, is your cory the only one in the tank? They thrive in schools so maybe getting a few more would bring him out of hiding :) Cory’s are some of my favorites!
  • fishygirl: Yes Colin is alone at the moment and I am wary of getting another until I have sorted nitrate levels etc.
  • Mcfish: Very well written and informative!!! Thank you and keep them coming!
  • WeeKian: Excellent Article! This sort of information should be published in all Aquarium Shops for hobbyist information. I am glad that I am in this website to learn from all the masters :)
  • Panthrmodern: Brilliant!! Very well written!
  • FishFan: Good.This helped me and a lot of other people a lot. Thanks.
  • Natasha: great question,isn’t 20-25% water change per week a little too much? i may be wrong but i have researched like crazy too with responses ranging from 50% per month (way too much) to 5% per week..just want a second opinion :)
    again great blog :D
  • Shocker: As for adding wood, I assume you mean driftwood and that can be a different animal altogether. Just FYI to anyone considering that.
    Great Post!
  • lstahlbrown: What an amazing list of mistakes. I only wished I would have read this before my fish got sick. I now realize how many mistakes I have made. I must say that Chuck is one tough fishie ....otherwise I would have killed him 3 years ago. I have done everything wrong. Thank you for compiling this list. I can relate with each and every mistake. I am learning as I go and I will try to make better choices for my fish. I didn’t realize how much I enjoyed them until breaks my heart to think I may lose one. Thank you for enlightening me!
  • Madeye: Luckily I came onto this site BEFORE I made all of the mistakes!
  • RollyPollyOlyy: great guide, helped me out alot
  • Harvey64Fish: You touched on a few things i did right and WRONG as a begginer, it was great reading, One thing i strongly agree on is research, I would not have had a platy fry problem if i had not acted on impulse and bought them becausu they looked so pretty, But its cool tho, I just had to buy another tank, unexpected like, SEE WHAT COMES FROM NOT DOING UR RESEARCH, my mollies just gave birth to about 10 or more fry, THATS WHAT I WANTED, gonna have to rehome some platty fry, keep some, will see if lps will take some off had, other than that, all is good, ON THE FISH FRONT...LOL
  • icamblue: Very Informative! A couple of things I have been guilty of... Here the other day I cleaned the filters in my canister all four in old tank water. I wasn’t aware of not cleaning all filters at once. I did only clean one half of my gravel and waiting a week before I clean the other half.
  • newaquarist: great summary, thanks, especially your point one about cycling and that pet shops dont promote fishless cycling. I did fishless cycling, but made a mistake and sparked a mini-cycle when I had fish - pretty hard to do these daily changes and see your fish suffer, so completely agree, please please do a fishless cycle and follow the other points.
  • kezza: Thanks for sharing, I remember just after having cycled my first tank for 3 weeks I went and purchased 2 x yoyo loaches . We’ll someone didn’t do their homework and shortly discovered my snails had been devoured. Cheers
  • ttarr2008: I thought it was good information. I don’t have a association with a pet store. But I understand and agree with u on what u said. The person that uses tap water and such is probably just lazy. And besides why not do it the way you say? Then you have no doubt that your fish will have a long and healthy life no matter what. I will do the extra just in case. It won’t hurt even if it doesn’t help. My mother always says that it is better to be safe than sorry. And I think it is good advice. I like u had a small bowl with a betta and now that I know what I am doing better so to speak I know I will enjoy my fish a lot longer. I love having them.
  • serralinda: I’m a new fishkeeper and just found this website tonite. I loved your article and sincerely hope I can find my way back to it for future reference. Great information, thanks.
  • LST: I have kept a school of cardinal tetra in a planted tank for almost two years.I change about 30% of the water twice weekly and do none of the tests for ammonia, nitrate and nitrite. The fish and plants can still thrive.
    So, I guess the tests are not a must.
  • Lizzy: Gosh, I too just wished I had read this before I lost 3 of my 4 fish. Great Information, that will help me and others who are starting out and want to keep their fish alive and healthy. Cheers!
  • Yolo_Dude: Lol u should make a post about how to prevent tiger barbs from eating mollies fins haha
  • Tan Soo Lim: Informative, especially for beginners.
  • Greybrooks: I’m truly grateful for this, and other, articles here.
    I had African Dwarf Frogs in college, and they were VERY easy to care for. I cleaned the tank once a week (100% water change, because I thought that was what was supposed to happen) and they lived fine. No filter...
    Now I"m realizing I probably made a HUGE mistake in getting Dwarf frogs now. The whole water, nitrates, etc......I never knew about any of these things.
  • cyn: An excellent mash of the information we need to know as beginners. I just started a month ago as well and its been such a learning curve...i’ve made many of the above mistakes. This site has helped me tremendously and I am trying to do good by my fish and rectify my mistakes :)
  • mollyluver: I remember when I was little and new to fish keeping, and doing almost all of these!! Brings back memories...
  • Goldfish Luver: Awesome blog!
  • Fishobsessed.: Well written and I have bookmarked it for future reference.
  • Rajeev: I recently bought a tank which came with a pump and mechanical filter media as well as the activated carbon. The salesman says I don’t need any ceramics for biological filtration, however I know this is relevant to have. I have fish in already for a few days now and they seem fine. Would it be too late to add the ceramic to start the biological filtration cycle?
    Thank you.
  • Betta Beauty: Very very sadly I have done EVERYTHING on this list at one point thanks for writing this
  • Benevolent Betta: Great blog! Very useful, I wish I saw this when I was first starting out.

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