Established tank, fish suddenly dying

Tank: 29 gallon, cycled 6 months ago. Plenty of ornaments/hiding places. Originally floated bamboo plants at top of tank (no purchased at pet store - Ikea)

Starting Population: 8 Danios, 3 Blue Ram Chichlids, 3 Spotted Cory Catfish.

During the past 2-3 week, six danios and one blue ram have died. Of the six danios, two stopped eating, got weakened, and died. The remaining 4 were very sudden as did the one ram chichlid.

I change and monitor the water weekly. All levels of ammonia/nitrates, etc are zero. I treat the water with de-clorinator with each change. I carefully keep the new water temp as close as possible to the tank water before pouring it in. In short, I do everything I've researched and read to do to keep the tank well. Currently, only the catfish are completely fine. The two remaining danios seem a tad lethargic right now, but they are eating. I think it could be the loss of all their friends? The two remaining rams are not eating and look like they aren't going to last two long.

I've used Jungle Parasite Clear a couple of times with no result. I can't seem to figure out what to do here. Any ideas? Also, do I want to hold off on introducing new fish to the tank until this problem resolves itself? Thanks.

Welcome to our forum.

Please don't put any new fish in the tank before you have corrected the problem. The tank was a bit crowded, but your careful maintenance probably eliminates that as an issue. It sounds like something toxic to the fish was accidentally introduced into the aquarium. What about that bamboo you got from Ikea? Is it possible that the bamboo or some other decoration introduced a toxin like varnish, dye or a preservative? Did you or someone else put anything into the aquarium a few days before the die off started? Did you add plants? Did the fish food go bad? Good fish food may stink a little bit, but it should not smell rotten or be moldy. A similar problem happened to one of our members, and he discovered that his little sister was feeding the fish.

I hope some of those ideas at least provide a clue. Please keep us posted.

There's a lot of odd things that can happen in a situation like this.

Was the outside of the tank cleaned, and perhaps some of the cleaner got in the water?

Is the tank in an area where someone might do hygenic things? Aerasol Deodorant? Hairspray?

These are oddball things that can affect your tank.

How old is your heater? Could it have a crack in it? Maybe be leaching an electric current into the tank?

Just random thoughts,

Keep us posted.

Thanks. I visited a new pet store today - non-chain place, guy seemed to know his stuff. He recommended adding Mardel Copper Safe as a start and see if there was improvement and also adding an air stone as there were a couple of them gasping at one point. I have a power filter and took out a little water to improve surface agitation and that seemed to help. I figured the air stone can't hurt and actually the two danios seem to love it.

Other than the bamboo plants, I can't think of any other way an outside toxin could have entered the tank. I have specific algae scrappers/buckets/towels/small bowls, etc used only for my tanks. I have a separate 10 gallon - home to a betta - and have not experienced any problems. I clean out/change water in both on the same schedule. Thanks for your help, I'll report back.