I have 9 neon tetras, 9 corydoras and 3 sparkling gourami in a 20 gallon tall tank with sand substrate and some plants from petco and driftwood. I've had the tetras since Saturday. I got the 9 corys and 3 gourmi in the mail yesterday and put them in the tank. At first, everything seemed fine. The corys were swimming to the top at times for air but were flying around and being active and then would rest for a bit. This morning I found 5 of my corys dead. The tetras are and gourami seem perfectly fine but the remaining 4 corys are pretty inactive as of now.