I've got a 37l tank, containing 6 cardinal tetras, 6 black neon tetras, and a yellow guppy all at about 2cm long. I'm sure this is overstocked (I did not intentionally stock it this way) and although the fish are happy and healthy as far as I can tell, and get plenty of exercise from the selective currents in the tank, I'd like to remove some to thin the numbers out a bit. I have a friend who may be using my old 24l tank to get some fish, and I'm thinking of passing over some of mine to her to keep- as far as I know, 6 2cm tetras is about the maximum you should put in a tank so small (is this about right?) and so I would give her my entire black neon shoal, but I'm pretty sure she wants something a bit more colourful, as they're pretty dull (as much as I personally love them). so, I'd give her 3 of my black neons and ask her to get some more neon or cardinal tetras to complete the shoal, but I'm afraid they wouldn't properly integrate together as a shoal. I honestly don't want to lose my cardinal tetras but if it's completely necessary for my fish's welfare, I will do. In my tank, the fish don't obviously move around in groups- both tetra shoals just sort of scatter around the place, and group together when there's threat (aka the gravel siphon) or food. they're overall really chilled; when I put my hand in to do something, they are hardly fazed and sometimes come over to kiss it. romanticised way of saying they're trying to eat me, I know. in conclusion, what do you think? would it be a bad idea for her to make up a shoal of 3 black neons and 3 of another tetra? and is my tank even seriously overstocked?