Any possible way to get a pic?
What are the water parameters?
Ammonia, Nitrite, NitrAte?
Clean, fresh water is #1 for good fish health.
Do a water change! 25% Then another until your water quality is great.
Is your tank cycled, have a big beneficial bacteria colony in your filter pad/ filter media?
If you need more information about this and fish keeping tools check the link in my signature for a very useful Aquarium Advice article.