We have had our tank for some time now. We started with fake plants then moved to real ones. (So much nicer). However, our substrate was not good enough for our plants and it was just getting nasty. So I changed the substrate. It is now a two layer substrate. I made sure everything was safe for the fish. Suddenly we started losing our neon tetras. This morning we are now down to 2 tetras after have 8 at the beginning of the week. We had 4 Julii Cory's, now one is not doing good. She is staying in the ground, and when she tries to swim her belly goes up. I checked all the levels of my take with the tests that we do have and the water is perfect. So don't know what to do. Please help!