Through much trial and error I managed to set up at 5 gallon tank with three guppies. I have had them there for over a year. I decided to upgrade to a 20 gallon tank. A friend gave me their filter Aqua-Tech 20-40. I set up the new tank, added gravel from my other tank and even moved one of my older filters from the older tank into the new one. I let that run for three days. Then I did a water change and added some of the water from my guppy tank into the new tank (about 2 gallons). I waited another 24 hours and did a water test. Everything looked fine. I moved my guppies in yesterday. This morning one was dead when I woke up and the other didn't last too much longer. The third one looked great and was swimming all around the tank, but now he too is just at the top of the tank not really swimming. I did another ammonia test and it is at 0. Anything else that I can do? I'd like to add zebra danios to the tank, but need to figure out what happened to the guppies first! Silly me thought the guppies would be fine in the new tank considering all of the measures I have taken (I also added quick start to the tank as well as I have done with my guppy tank in the past) and I emptied the guppy tank, so I can't move him back now. . . .