Hi everyone. I am a 4th grade teacher, and in past years had (2) 10 gallon tanks in my room for guppies. The kids love seeing the complete cycle of life. This year, I used a grant to get a 40 gallon tank for my room. I got everything ready to go and started a fishless cycle. Once that was finished, we added our fish. Everything was going well. We had new babies, pregnant females and no deaths. Then, over the weekend a few weeks later, every fish died. Friday they were fine, monday dead as a doornail. I thought maybe the tank got too cold (it was our first cold weekend), but the temp read 76. I also used my api test kit to check the water. All levels were normal. I drained the tank and then filled it again. Used water conditioner to treat it. We added some more guppies (young and mature). Over the next few days, most of the mature, and even a few young ones died. I checked the water again...normal. So I did a 60% change, treated the water and got some more. I had them for 2 days and already two new matures have died. Most of the fish (young and old) are close to the top. I've never had a die-off like this before and don't know what to do. Any suggestions?