About a month ago I bought my boyfriend a 5.5 gallon aquarium, it has a small drip filter built into the hood and I also purchased an air pump and a heater. For substrate I used black sand from the petstore that I washed, decorations are a fake piece of driftwood and several clumps of plastic grass. Temperature is 78 degrees, pH is 7.0 and other parameters are within normal ranges. I let the tank cycle and then purchased two small guppies for the tank. These died in 12 hours, took them and a water sample to store for testing and everything was normal. Bought 2 more guppies, and these died also. Took them back, did a water change and waited a week then tested water again (which was normal) and bought more fish, these died. Went to a different store and bought some ghost shrimp and 2 more guppies. Guppies are dying, the shrimp are doing quite well and are even molting. The fish sort of hang out at the top of the tank like they need air but don't gulp, and then they sit at the bottom of the tank and die. Anyone have any advice? I am not a novice fish keeper and I cannot figure out what to do. I am about to rip my hair out and just scrap the entire thing. Everything appears normal, nothing is out of the ordinary. Fish just keep dying.