first if you could define "baby tank", this will help
but I will assume the fish are either juveniles or fry
even though I frown upon chemicals in the tank, if you apply Seachem Prime chemical remover at each water change, the ammonia (and nitrite and nitrate) levels will go down to zero. Prime is a little expensive, but when I first got into this hobby, I had all kinds of contraptions in the tank and used all types of chemicals, and the only chemical I wound up using before I eliminated all chemicals was Seachem Prime with each water change
on the filtration, it's okay to have too much filtration, but if the filtration robs the nitro-bacteria the chance to continue its nitrifying cycle, this could be the culprit.
May I ask how long you've had this tank up and running?
Meanwhile, go buy a bottle of Prime, do your water changes, use it for two weeks, then let us know of your results