Hello; One thing i do a bit different than some has to do with cleaning power filter media. I do not rinse old media in used tank water. I keep a supply of new media on hand and simply replace the media. Been doing it this way for a few decades.
Now to the objections some have for this method. For some reason there is an idea the biofilm of beneficial bacteria (bb) only exists on the filter media. I do not find this to be so. We can run tanks without any filtration or only simple sponge filters. The bacterial will colonize many surfaces in a tank. Likely more colonies on surfaces near moving water. So, yes there will be some biofilm of bb in the media. There will also be such a film on all the plumbing and housing of that filter. The bb can be on the glass and other surfaces inside the tank. Some folks get ceramic or plastic "bio-balls" for a tank the purpose of which is to provide massive surface areas for the bb to colonize. I have used glass marbles in a large HOB I had back in the 1970's.
I run HOB filters currently (hang on back). I generally change the filter media on some sort of schedule or when the media starts to clog. I do not disturb the slimy biofilm of the filter box or its tubes. The media traps detritus and the organic part of that detritus will rot in the tank if left. I prefer new media over clogged up old media. That media is part of the mechanical filtration and is meant to be replaced.
Note - I do not use the somewhat costly replacement cartridges sold for my filters. I buy rolls of filter material, cut it to fit and rig a way to add it to me power filters.
To your fish troubles. The only options are to get rid of the fish or do large water changes (WC) every couple of days. Being these are native fish and it could be illegal to keep them in a home tank you may want to check the laws in your state before contacting a fish shop. One thing you should not do is to release the fish back into the wild. They appear to be diseased to an extent or may have picked up tropical parasites for the other tanks unless you practice using nets and such for only one tank.
One thing you can try which may help is to reduce feeding. excess uneaten food will sour a tank. Have a day or two a week of fasting combined with lots more WC.
Good luck