please help!!

i really don’t know what to do at this point. my fish is about a year old. i have two of them in a 20 gal which i know isn’t big enough but i don’t have the space to put them in anything bigger right now. the parameters are perfect (i use the liquid test kit) my other fish is fine, my plants are fine, but my fish will not eat. he can swim down, but is only staying at the surface. my other fish is perfectly fine. i tried giving the sick one little bits of a boiled pea because i thought it could be a swim bladder issue, he just refuses to eat it. which is very out of the ordinary. i’m so worried and i’m really not sure what to do. his fins and gills and scales are fine, there’s literally nothing out of the ordinary other than he looks a little thin as he hasn’t been eating for two days. please can anyone help me?