Hi, and welcome. This is the wrong forum for this question so a Moderator will probably move it to the correct forum. In the meantime can you tell me more information on the tank?
--what size tank?
--how many guppies do you have in the tank?
--have you tested the water for ammonia, nitrite, nitrate?
--What do you feed and how often?
--What is your water change schedule (how much and how often)?