Hello and welcome to the forum,
Like the other members have mentioned:
How long has the tank been setup?
Do you own a test kit?
What are your water stats(ammonia,nitrIte,nitrAte&pH)?
Have you cycled your filter?
size of tank?
Tank occupants?
tank temp?
what chemicals do you use(e.g dechlorinater)?
If you have not cycled your filter i would recommend performing a large water change/gravel vac, at least 75% of your tanks water volume in order to lower the levels of toxins(ammonia/nitrIte)in your tanks water.
The more information you provide the better the chances are one of the members will be able to identify whats going on. You have come to the right place in regards to recieving expert friendly advice.
Keep us all posted, Keith.