Poorly Trilineatous Cory

Hi guys,

So ive been on holiday for 3 weeks so mum has been looking after the fish tank best she can for me. She has been checking ammonia daily for me and doing 50-75% water changes every week and adding prime on other days as im still having a few ammonia issues :/.

The last week she noticed one of my corys isnt so happy I was wondering if anyone has any ideas for whats wrong, he is very discoloured and appears lifeless alot of the time. Wondering if theres anything i can do or if its too late for this guy?

I left my mum food to give them every other day to include mixtures of vitalis catfish pellets, fish science cory tablets, hikari sinking wafers and tetra micro granules and tetra min flakes for the other guys..they haven't had any frozen food for 3 weeks as I didn't want to bother my mum with it. The other corys I have are all fine  This guy doesn't seem interested in eating anything he mostly just sits in one place 

It does have barbels, think its just a bad picture, I have fine sand substrate. I wasn't sure if he was sterbai or trilineatous as a sterbai picture I saw looked very similar too 🤣

I still have some melafix left prehaps I will give that a try then.

I have used the melafix before with pimafix to try and combat the possible fungus i had, no bad effects with corys although it also didn't fix anything :/