Help! I'm new to keeping fish. I have a five gallon tank. There were a few cherry barbs, and then I added one guppy. The guppy died yesterday, and I simply assumed she didn't "adjust" to the tank, as I had only bought her the day before. I examined her, and the only thing I could see was that her mouth was open, as if she had been gasping for air. None of the other fish were having trouble breathing, so I ignored it. But today, I found one of the cherry barbs dead, with the same open mouth! Any ideas what the problem is? I use filtered water, with "Chlor-out" added..the water had been changed earlier in the day when the guppy died...and the cherry barbs have been living quite successfully, up until now. Unfortunately, I have nothing available to test my water with, but I can't see how it would be my water when they've been living just fine till now? Could something have happened when I cleaned the tank? Please help! Thanks so much.