Looks like you might have a similar problem I had that killed 90% of all my fish and wiped out three years of fish growing.
Me think’s that you might have some bacteria problems in your tank, or filters that are harming your fish. Looks like you will have to Nuke your whole tank, (But only it all other attempts fail.) and the problem persists.
How to..
First you should fill a clean bowl with fresh treated water and move all your fish to that with a net or something, limit the transfer of water from your tank as this may be contaminated. Clean the heater and place that in the bowl.
Then empty your tank and throw the plants away. They probably soak up the bacteria and hard as hell to get out.
Now throw away the gravel at the bottom of your tank and get new pack. Gravel is extremely hard to clean properly and its much more safe to get new bag. Objects like logs stones need to be scrubbed boiled in hot water and microwaved for five minutes.
Your filter is the next that needs to be Nuked and Nuked HARD.
Dismantle the pump and remove everything inside, Scrub and place filter sponges in the microwave for a ten or fifteen seconds until it steams. Be careful not to melt the sponges. Anything plastic will also need to be microwaved, but not the metal components or the pump unit itself.
If you have an external pump, microwave the trays and sponges, assemble and then run boiling water through it for half an hour. Basically everything needs to be killed inside the pump.
Once done, rinse everything and make sure the tank is scrubbed to the bone, don't use soap.
Then fill the tank with treated water, preferably to normal temperature and make sure the pump is working normally.
Now restore the heater and objects back into the tank and make sure everything had been in the microwave beforehand. Once done, move the fish back in, but transfer them with the net again, not pour the fish and water into your clean tank.
Normally this would reset everything back to normal, your fish might be okay after this, some might die, but the next step will need you to keep an eye on everything as your filter is not set-up with bacteria and there is nothing to control the toxins in the water the fish make:
Keep an eye on the water toxicity and if levels rise too much, add a treatment called Prime. It should detoxifies everything. Also there’s an additive that instantly establishes a bio- filter, but I forgot what It’s called.
With my tank, I left everything to stabilise on its own and the only fish I lost was the large Clown Loach. I think that died when I treated my angelfish for white spot, they don’t like medication.
