I'm basically on the same lines as others, sort of. If you only want neon tetras and cories, I would increase the cories to 20-25 (assuming this is a 4-foot tank). And you could easily do 70-100 neon tetras. Don't need that many, but the option is there. Also assuming floating plants. If you want other species, 20-30 neons, depending what other species. Cories are substrate fish basically, and neons tend to be mid-lower level. So you will have the upper 1/3 to 1/2 of the water empty with just these two. Hatchetfish? You have the space for a good group of the larger species, like the beautiful Platinum (Thoracocharax stellatus).