I have a 29gal with what was 6 small assortment of fish that got along and was approved by a highly rated fish store (tetra, platy, and a shark/catfish guy). However, after having them about 2 month, one by one they are slowly dying off
the shark was the happiest, energetic, most peaceful fish as of yesterday and I got home today to see him swimming side ways and will stop moving for a little and float around, then struggle to swim correct again (he just looks sick and losing color and I know he'll be dead soon.) I used a master tester and all my levels are with in normal ranges and I do regular partial water changes/gravel sweeps... What could be wrong?? Please help anyone :[ I want to save the remaining fish.