yikes.. I would get a test kit to see what is in your water.
Yeah do some reading on the nitrogen cycle in a fish tank
Basics: Fish poo and pee which has ammonia in it. Ammonia = bad for fish
In a new tank there is nothing to remove this.
Over time bacteria will grow that turn this to nitrIte, nitrite also = bad for fish. Bacteria also grow which turn this into nitrAte. Nitrate = OK for fish in lower levels.
Regular water changes of about 20% help keep this low.
The bacteria grow in your filter and substrate. Never clean all your filter in one go and never use tap water as the chlorine kill the bacteria, use tank water from a water change.