I couldn't find a very adequate answer while searching the forums on the rules for schooling fish. We have a 10g freshwater tank in the process of fishless cycle and we've been researching what we want to put in there. We're probably going to start with a school of 5 tetra or danios. There are soooooo many varieties of each kind, it's hard to pick just one. So I was wondering, can you mix and match to achieve the complete school of 5? I assume you would have to pick varieties that are similar in shape and max size. Or is it just a no no, a school is purely a number of fish of the exact same variety? I saw two or three different colors of skirt tetra (just an example, we'd probably go with a smaller max size variety) at the store and thought it would be easier for my 5yo daughter to name and bond with her fish if she could tell them apart, but I don't want to mix and match if it is bad practice and the fish won't be happy together.