I have a 55 gal tank that is 2 months old. I have 2 black skirt tetras (silver dollar size), 5 zebra dannios (2 inches long), 2 smaller glowfish dannios, 1 dalmation molly, 1 bristle nose pleco, and three frogs. I have added just a few fish each weekend for the last 4 weeks, and we've only lost one angelfish early on. Last weekend, I added 6 cardinal tetras, which are maybe a third of the size of the zebra dannios, and 4 have died/disappeared. One we found, with it's head missing, about two days after putting them in. The other three just disappeared, one per day or two days over the course of the week. I had my water tested by the aquarium store, who said it was good, and my own test strips show it in the safe range on everything. It is hard water and alkaline, but within the safe range. The temp is steady at 78 or 79degrees. So I'm wondering, is someone eating them? Maybe the zebra dannios? Or the frogs (who have grown suspiciously large very quickly)? Or are they dying and then being eaten? I would like to save the remaining two. Should I return them to the store? Thanks for any input!!