Hi there and welcome to AA!
If you haven't done so already, please test your water for ammonia, nitrite, and nitrate and post the results in numbers (ppm) to the best of your ability (those color charts can definitely be hard to read). How long has your tank been set up? When was the last time you added new fish? What is your water change schedule, and what percentage do you change? What size tank do you have, and what filtration are you using? What is your stock list?
Sorry for all the questions, but any answers you can provide will help us get down to the bottom of the issue.