rainbow shark health

I’ve had a rainbow shark for 3 days now.. he still hasn’t turned fully black and he barely swims around. He’s in one position floating for hours and I’m concerned something is wrong. I’ve seen him eating algae but ever so often. My tank is 25l (I am aware this is small, I will upgrade to a...

Hello :) I'm new to fish forums, but I've used the info from your forums for years. I decided to join because I a puzzling rainbow shark. I've had him for about 6 months now. When I got him he was very small, probably the smallest I've ever seen. He was pretty pale when I bought him, but I...