What are your water parameters - Ammonia, Nitrite, Nitrate, pH, temp, general hardness/GH, Alkalinity/KH,
What size is the tank?
When was the last water change?
How long has the tank been set up, and what fish and their sizes are in the tank?
Read the link for the new to fish in my signature it will give you extremely helpful information.
Get Prime water conditioner, watch your feeding so it isn't too much.
I would do a 25-30% water change with temperature matched water, and dechlorinator. Then do another 30% water change pretty much right away.
Save a cup amount of tank water at the beginning before you change the water and mark it as the old water so you know. Test before and after, the water changes.
Fish can get aggressive when the tank parameters are off of a safe level and they can fisht if there are too many fish in too small of tank too.