Soo I went on holidays for a week and left two feeder blocks in my tank (a super market brand but not one I've tried before). Anyhow, I got back and did my usual water change 20ish % and gravel vac. I noticed that my largest shubunk was not swimming as much and staying in the one spot, she was eating normally for about 2 days.

Last night and today she was sitting on the bottom gasping and not eating or moving. Before that she did a massive I'd say fish diarrhea just once. She seems swollen/ bloated and her tail fin is I don't know how to describe, limp? She's attempting to swim and move but it seems to tire her out.

Things I've tried:
Water test was
ph 7
Ammonia .25ppm
Nitrite 0ppm
Nitrate 20ppm (tested nitrate before and after water change and it's about the same, usually it's 0ish)
50% water change with more gravel vaccing under ornaments and stuff
Double dose of Prime
Feed some peas but her tank mates gobbled them.

All of the other fish are doing very well eating and behaving normally, my apple snail also died at some point within the week.

Any help? I hope it's not something horrible like dropsy, she doesn't look pine cone, nitrate poisoning maybe?

*cries*

She's gotten much worse. She can't stay upright any more and won't try to swim. I've prodded her every so offten with a net to encourage her to swim but I think she's given up. The other fish are still fine, it's so heart breaking, she was a rescue fish and I feel bad :(

Pics from just before.

when a fish is resting on the bottom of the tank in a listless way or on its side or half-side at the water surface...and/or not taking any food, then yes, the fish will probably die

fish do two thing best: swim & eat (and of course mate)...but the first two signs of an ill fish is non-movement and/or not taking in food

reason one must observe every day, several times in a day to notice signs of any problems as to prevent a serious illness from happening

sadly, that fish of yours was way too ill to even scurry to the food...in these cases, it's a matter of hours before they die

best thing to do is, place the fish in a small dish of water, place it in the freezer, and in about one hour, the fish will have died (if not sooner) or place it down in a small dish of water with clove oil in it, this too will euthanize the fish without any pain