Hey folks, first time here. Looking for advice for my fathers fish!
So around 5-6 years ago my father bought 2 fish (He doesn't know the name of the breed but you can see them in the photos) and a tank that, at the time, was large enough. He then invested in 2 Black Moor fish, and it's one of these Black Moor's who is the star of this post. Over the last couple of weeks I had noticed he had been sitting at the bottom of the tank quite a lot and I found it quite alarming, he doesn't really move, and when he feeds them the lil guy sort of shuffles around the bottom of the tank bottom feeding. He/we know almost nothing about keeping fish (Being honest) so we are looking for some advice. Here's some info that you may require, if you need any more please let me know.
He cleans tank out once every 2-4 weeks and replaces 2/3's of the water. He has one filter (that you can see in the photos). He doesn't know the size of the tank (Which I believe to be the main issue), and he also doesn't check the nitrate levels within the tank, which has been suggested in a few posts I browsed before posting. This morning he imposed "Social distancing" on the affected fish by moving him to a smaller separate tank.
So yeah. He does do his best, and he really does care about the fish, so don't be too hard on him, haha! Any advice would be more than appreciated, tanks in advance!