Could you please supply us with as much of the following information as possible. This will allow us to give you better information on resolving your problem

Size of tank in Litre or Gallons, if you don't know the water capacity then the physical size of the tank L x W x H?

What filter do you use?

How long has the tank been set up?

What water tests have you done and what are the readings pH, Ammonia, NitrIte, NitrAte, Phosphate, water temperature?

When was the last water change and how much did you change?

A list of all the inhabitants?

When did you last add fish, plants, ornaments or other tank equipment?

Have you added any medication?

Can you post a photo?

The more you can tell us the more we can help