09-03-2010, 07:25 PM
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You have room for both, but as mentioned already cory species do tend to shoal together by which I mean the fish of one particular species. Shoaling means they need to have a group of their own species for various reasons; while a group of 5 works very well with corys, 3 of a species will be OK along with 3 of a second species. Some species will shoal exclusively with their own species, others will now and then mix. In their habitat, more than one species is frequently present but each species numbers in the hundreds of fish so "mixing" of species is not really the same as what we intend with only a couple of each species.
My pandas are almost constantly together, and I have 3; I also have another species in the same tank, and sometimes one of the panda will swim with one or more of them, but not often. Other memebers have had panda that mix with other species more. The bottom line is that each species needs 3 or more to be secure.
Panda and bronze are a good combination; they are quite different in appearance, plus they both do well at cooler temperatures (low to mid-70's F) which also suits the zebra danio. A temp of 75F would be good in your 20g. You can read more info on all these fish in the profiles, second tab from the left in the blue bar at the top. Corys are under catfish, zebra danio under Cyprinids.