What’s wrong with my fish ?

i brought this fish on Christmas Eve out of pity and even though he’s nothing special he has quickly become my pride and joy. He has been pretty clamped up and lazy since I saw him in the shop and I figured he would open up when he got used to a new tank but today he’s gotten more lethargic and just lays around on the bottom and breathing quite heavily and looking incredibly weak when trying to swim. I’ve caught him laying partially on his side and it seems to be getting worse.

I have him in a 60l tank that was set up and cycled back in the start of November and that I am currently using for growing plants to put in my other tanks, he is in there with 5 bronze Corydoras, 3 hillstream loaches and about 7 guppy fry as I didn’t want him to get bullied. The tanks temp is 25.5°c

I am currently using an anti fluke and wormer with the second dose due tomorrow I followed advice and did a 75% water change, sand clean and filter clean 24 hours after the first dose and will do the same after the second dose tomorrow.

I have a master test kit being delivered tomorrow so I will include water parameters when that arrives

if there’s anything you can recommend to help then it’d be greatly appreciated

You can try feeding him more often (3-5 times per day), but I don't know if it will help considering his current condition. Once fish lay on the substrate and don't show much interest in food, it's usually too late to save them.