we bought new 48 litre tank about 3 weeks ago and let it settle for 4 days we then added a small fish which later died, we took the water from the fish tank to be tested and was found to have ammonia in it,we was told to empty a quater of the water a 4 days and then try again which we did and still amonia in it told us to do the same and clean the filter in the tank water which again we have done and still amonia can you please help us.since the fish has died therehas been no fish in tnhe tank.