I have a 29 gallon tank with three bristlenose plecos and three goldfish( two telescopes and a fantail). The tank is fully cycled, I have had it for about a year, but within the past two weeks or so the water has gotten cloudy,which has happened before, and I added algea remover to get rid of it which it did the previous times. This time however all the goldfish have died within the past week they grew this white cotton stuff and the same material started floating around in the tank. I did a 50 percent water change, cleaned and changed the filters, and added some medication because one of them started losing scales and had red wounds. The plecos are behaving fine and have none of the stuff on them. What can I do to fix the tank chemistry to save the plecos? And if nothing can be done for them what can I do to clean the tank and try to start over? All the chemical levels are normal except the ph fluctuates between 7.8 and 8.

gez dont know, ask PK.

Usually when your tank gets cloudy it is trying to cycle again. I do not recommend any chemicals including algae remover. You are just defeating the purpose. Not sure what filtration you have but it sounds like you are getting rid of the good bacteria and the tank starts to cyle again. How do u clean your filter?

just rinse it in the old aquarium water during changes. i do a 25% change once every 2 weeks.

Gee thanks for the props mor2you2!

Been busy.....

Sounds to me like the bio-load in the tank overpowered the capacity of the biological filter.

At first, when the tank was new and the fish were small and didn't eat so much, there was enough benneficial bacteria in the tank to seed the filter after each filter cleaning.

The way you describe the goldfish, and the fact that they all died, sounds like ammonia and or nitrites built up to a toxic level.

Maybe the products from the waste, ie food, fish poop, etc,(chemicals as you called the water peramiters) are normal now, but I doubt they were normal before you did the water change.

Plecos are pretty tough fish, so that's probably why they lived.

I would either get a bigger tank with a bigger filter, or add another filter to your tank. You can clean one filter while preserving the biological (benneficial bacteria) filter in the other. Then, by the time you clean the second filter the first will have built up a colony of bacteria.

I would also try to feed less. Most fish will over eat, and they don't need that much food, and excess food will only polute the tank.

Also, do water changes more often. The end product from the bacteria doing their stuff is nitrates, with an (a). Nitrates are less toxic than ammonia or nitrites, but reach toxic levels pretty quickly in some tanks.

get a nitrate test kit and do water changes enough to keep the nitrates below 20 ppm.

Keeping the biological filter hapy, feeding less, water changes to reduce nitrates.........This should be good to keep your fish alive and healthy.

Also, the cloudy water could either have been algae from excessive nitrates, or it could be the cycle starting again as beachy woman said.

Good luck! PK

Sounds like you understand more than many about doing water changes and rinsing your filter in aquarium water. I didn't mean to aasume that you were a total novice.

I still think that as your fish have grown you've pushed the capacity of your biological filter to a point where toxins are killing your bacteria.