wow...
so how do you clean your filters? Any chance you killed the cycle that way? When cleaning filters make sure you using old tank. Water changes don't kill cycle even 95%, the bacteria (good stuff) lives mostly on the filter media.
As far as getting back on track I would say you need to be 70-90% wc daily 25% is simply not enough. Feed only once a day, very small amount to see if anyone is interested, if not siphon out the food after 5 min.
no meds are needed here. sounds like its simply a water quality issue / cycle issue. I would keep the lights off during this time to promote resting and not further stress them out. also make sure remove all that ornaments and give the substrate a clean. add a bunch of floating plants which will help improve water quality, every little bit counts. Add only a few decor (things for the fish to hid in but nothing excessive till they are healthy. You need to be able to see if someone died or how they are doing in general. With too much decor it could hide dead fish or fish waste which is bad.
Keep temps mid 70's too high and it makes it harder for them to breath.
Add air stones if you can, keep high surface agitation. prime is the best dechlor, use it.