water change regimen (DOCs)

Since 20% weekly water change is not supposed to equal or exceed the accumulation of DOC's, I'm wondering how some of you are combatting this for long-lived tanks. My assumption is that the solution lies in supplimental/additional water changes but I'm not sure of a good regimen.

Would a good regimen look something like:

20% weekly and once a month 20% bi-weekly water change (as long as it does not coincide with filter cleaning).

or 20% weekly with supplemental water changes every 6 months?

Something totally different?

Sorry to respond to myself, but a mathematical approach would be more appreciated than general rule of thumb - but all impressions/experiences appreciated.

Previous posts follow.

Correct frequency of water changes/vacuums depends completely on the tank's bioload, the type and amount of filtration, frequency of feeding, amount of time the lights are on, position in the room, etc. etc. etc. Here on the newsgroups and in most forums, weekly is recommended; in most printed literature (with notable exceptions) monthly is considered adequate. Those that are true experts *seem* to lean towards monthly, but people that are running businesses (or have in the past) raising/breeding/selling are saying weekly. My stand is that each aquarium has it's own personality and it's impossible to make a one-size-fits-all rule

When I say 'mathematical approach' I'm thinking basically about the relationship between bioload and waterchange somehow. I can see how feeding, at the least, would definitely through a monkey wrench in any attempt to do that. Unless there's someone who's figured it out/accounted for it? [no grandkids ;) ] Of course being able to run an appropriate & accurate test makes this formula irrelevant.

Personally I do 40-50% weekly on my tanks. If you get a protein skin on the surface you can float a paper towel on it and toss the paper towel as a good method to get rid of some of it.

Well I'm leery of >20% water change causing too great a change in osmotic pressure, etc. as well as having to preheat all that new water/avoid tank temp drop. I'm also trying to not use a protein skimmer as I have freshwater tanks.

George wrote,

Somewhere sometime ago, I read that 'a weekly 20% water change leaves about 30 days of accumulated DOC pollutants in the tank'. I ran a few test on my own and agree, IF one only does water changing. I found that #1 - pre-filtering (and keeping them clean) makes all the differences in the world. #2 - Removing any solid waste from the substrate (gravel vac). #3 - Live plants (the more plants, the less DOC accumulation). #4 - Wet/dry filtering (I'm sold on the bio-wheels), all really help to remove DOC. High performance filters are not necessarily the best filters for removal of DOC!

With a chemical hardness test kit and a conductivity meter DOC can be determined by comparing the differences between the two test in ppm. My tap water, depending on the time of the year, has 150ppm to 200ppm TDS. Spring water normally contains less than 500ppm TDS (total dissolved solids)............ Frank

: Umm so is Nitrate a good indicator of DOC accumulation? I guess that : makes sense but I thought DOC's aren't measured by standard test kits : esp PH, Ammonia, Nitrite, Nitrate. What's the best way to monitor : this?

For around $40-60, you can purchase a total dissolved solids (TDS) meter. Usually they're combined with a pH meter or conductivity meter as well. While TDS would also include minerals, assuming that is fairly constant from your starting water parameters, you can monitor for an increase in DOCs via watching for an increase in TDS. This does not work as well in tanks with calcium carbonate substrates, but is still a better indicator than nitrates, which can be consumed by plants and anaerobic bacteria. I've got a tank that constantly sits at undetectable nitrate levels even though the tap has 40ppm due to plant load. Doesn't mean it doesn't need water changes.

: When I say 'mathematical approach' I'm thinking basically about the : relationship between bioload and waterchange somehow. I can see how : feeding, at the least, would definitely through a monkey wrench in any : attempt to do that. Unless there's someone who's figured it : out/accounted for it? [no grandkids ;) ] Of course being able to run : an appropriate & accurate test makes this formula irrelevant.

I doubt anyone has sat down and done mathematical calculations to determine optimal water changes. You might want to peruse the archives at thekrib.com because if it has been done in the past, that would be the most likely site to have it. There are those who just do large water changes (40-50%) every week to be on the safe side or because they have high bioloads. I will admit I lean towards this school too, but mostly because I get so preoccupied in cleaning the substrate that I forget to keep an eye on how low the water line is dropping until I hear the filters splashing. My rule of thumb is a dirty substrate means the tank still has more organics in it than when it started. It's by no means a mathematical approach, but it is a common sense one and that's sometimes sufficient, particularly when it comes to water change schedules.

But aren't large water changes & deep gravel cleaning a shock to the fish and biological filtration, etc?

I did also and it was the inspiration of this thread. Appreciate your and everyone else's input & tips.

: >tanks with calcium carbonate substrates, but is still a better indicator : >than nitrates, which can be consumed by plants and anaerobic bacteria. : >I've got a tank that constantly sits at undetectable nitrate levels even : >though the tap has 40ppm due to plant load. Doesn't mean it doesn't need : >water changes.

: Part of my confusion is my local tap water has high nitrates, I'm : starting a planted aquarium, and plan to continue experiments with : products like NItra-Zorb etc. It seems like you can easily scew the : results in a standard test kit to look good whilst other toxins : slolwly creep up.

Yes, since the standard aquarium test kits only measure a small amount of the spectrum of water quality, it is inadvisable to rest full on one result such as nitrates to determine water change frequency. I was involved in a debate with someone over one of those 'miracle in a bottle' products that kept his nitrates at a constant 40ppm. I was trying to explain to him why this did not mean he could stop changing the water on the tank, but he just did not understand it. Glad to see you do understand it. TDS is a bit more reliable than nitrates simply because it measures so much more. It's hard to make a determination of what exactly is causing the TDS to rise without laboratory kits, but if your KH and GH are remaining steady while the TDS is rising, that's probably a sign of DOC accumulation and means it is time for a water change.

: >because if it has been done in the past, that would be the most likely site : >to have it. There are those who just do large water changes (40-50%) every : >week to be on the safe side or because they have high bioloads. I will admit : >I lean towards this school too, but mostly because I get so preoccupied in : >cleaning the substrate that I forget to keep an eye on how low the water : >line is dropping until I hear the filters splashing.

: But aren't large water changes & deep gravel cleaning a shock to the : fish and biological filtration, etc?

It all depends on the source and tank water. If your source water is close to the tank water in terms of temperature, pH, KH, GH and TDS and you have adequately treated the source water to remove chlorine and chloramines (including binding the ammonia), then large water changes are usually fine. I even do that with Tanganyikans, which have a reputation for not tolerating change. Where people run into problems with large water changes is when they are not using the proper water conditioners for their tap water, such as using sodium thiosulphate with chloramine tap water which only breaks the ammonia-chlorine bond and neutralizes the chlorine but does not bind the ammonia, or if their source water parameters vary greatly from their tank water parameters. Deep gravel cleaning should not affect biological filtration unless you're using an under-gravel filter. Substrate holds much less bacteria than filters. Now, if you haven't deep cleaned in a while, the substrate can go anaerobic, which can release hydrogen sulfide when disturbed. In such cases, cleaning only a small portion of the gravel at a time is advised so one does not release toxic levels of hydrogen sulfide.