If this is a new tank.....did you cycle the tank first? This sounds like the tank is starting to cycle and the ammonia has peaked. This will cause major stress on your guppies.

There are plenty of threads on here about how to cycle your tank, or do some research either on line or go to the library to find some books or videos on the subject.

The best thing I can think of at this time is to do a 50% water change every second day for about two weeks, then go down to water changes twice a week......and after a couple weeks doing this....go back to regular weekly water changes. You may want to also use a good water conditioner available from you pet store, to remove chlorine and or chloramins....this depends on you water. Your pet store will probably be able to recommend which one you need.

Good Luck!

thanks i'll do that!!!