I'm back again, having problems with the same fish...
He's erratic and gasping for air, banging against the glass and decorations, kicking up sand and unearthing plants. Last time he got like this, the oxygen in the water was too low. But I keep the filter on at all times, making sure it agitates the surface, the temperature is fine, they're fed regularly and I tested the water it looks okay.
It's horrible seeing him like this I don't know what else to do.
He was given to me as a "rescue" sort of, so I wasn't prepared to have a fish this big but thought it woud be fine. I recently learned this tank is way to small for him, could that be the issue??
It's not overstocked, it has a few neon tetra and plenty of hiding spots. It's 58 Litres, and he's about... 10 cm long?
I can't afford nor have space for a bigger aquarium right now, so my best other option would be to sell/give him away to someone who is more equipped to care for him. But I can't know for sure if that will make him better?
TLDR: could the aquarium being too small cause this? Do I give him away?
EDIT: Thank you so much to everyone for your help, I didn't expect this to get so active I am doing my best to act on the advice and answer comments at the same time. so far I have turned off the lights, added an air stone, unplugged the heater, checked for stray current but as far as I can tell there is none. I am moving on to do a roughly 40% water change right now. and after all this is done I will make a public post on a local forum to rehome the fishes. Thank you a lot for your help, I will try and answer future comments as well.