Black molly sick after cleaning tank

[nicole0701]

Hello. I recently cleaned my tank, it's a 20 gallon with 6 mollies (4 of which are fry from the original two that have grown) and a dwarf gourami.

Had the tank for about 2.5 years and no problems. I am pregnant so have not had the time to clean the tank unfortunately in a long time until now, although the fish were still healthy and ammonia and nitrate level were fine. The filter cartridges were so dirty that I replaced them with new ones, then waited a few days and cleaned the tank, about a 50% water change since it had been so long. I also took out some of the plants and put in new ones.

Now my black molly does not seem to be doing well. His fins look split in certain places and has a small white spot on his mouth. All other fish are fine; however, ammonia levels are at 0.25 ppm, and nitrate levels are between 40 and 80 ppm (always hard to tell with API color chart).

I am worried I upset the balance of my tank by cleaning it too much and removing the good bacteria. If that is the case, does anyone know how long it will take to reestablish the tank or if there is anything I can do to help speed up the process before any other fish get sick?

Thanks for any advice.

 

[sinibotia]

You never, ever want to replace the cartridges in your filter. That removes a huge portion of the beneficial bacteria in your tank and is likely at the root of your emerging problems. Your nitrate is also incredibly high.

If you haven't cleaned the tank in a long time you also may have done more harm than good by changing parameters too quickly. After being untouched for a long time, water chemistry tends to shift heavily in certain directions. These shifts can kill fish by themselves, but doing that water change may have been essentially a reverse shift- which can be just as dangerous.

The best thing you can do at this point is to keep an eye on your fish and keep doing small water changes. I wouldn't do more than 25% at a time, but try to do them every single time you have ammonia show up on a test, until it is no longer registering and your nitrates are in a safe range. This shouldn't take more than a week in all likelihood.