Ive kept tankbusters, stingrays, and shovel cats. Ive also had way to many plecos in a tank for breeding.

If you have the recomended filtration for the size tanks you have, double or triple it. Ive used carbon and charcoal in the filters with good results on crowded tanks but never anythin to absorb ammonia.

Koi, goldfish, stingrays, and most catfish are all poop machines and mechanical filtration is best and alot of it.

If you not on the second floor of your building consider an indoor pond.