They'll do just fine. I'd advise against play sand though as it tends to compact and it's just messy.I've never kept cories before, but for a while I've been planning to do a 20 long for guppies with just play sand and no other plants or decorations. I thought cories would be a nice addition for the bottom, but obviously not if they won't be happy. I know most fish prefer some sort of cover, but how vital is it for cories? If they won't do well, is there any other bottom dweller that would work in that tank?