Was your tank cycled when you put the fish into it. Guppies are not good fish to use (or any fish for that matter) to cycle a tank with. The ammonia levels can kill fish very fast.
Do you have a kit to test for ammonia? If so would you please do a test for ammonia in your water and if it exists in the water you need to do a massive water change to bring the ammonia level down so your fish are not being poisoned with their own waste products.
We will also need levels for Nitrates and Nitrites if you can get them for us. I thank you.
I am so sorry for all the trouble you are having I do not know a lot about guppies and their lives but I do about the Nitrogen cycle and it sounds like what the problem may be.
Rose
