My thought is that the nitrate spike may have caused some more long term damage, how long and how high were the nitrates?
Are you crazy? Clowns are deadly because they take the stinging cells from the sea anemones they sit in and use them as protection. Even if the clown you gave him as never seen a sea anemone they lion might of still know it could mean death to eat it.I've tried mollies and even a clown, and nothing has caught his attention yet, and the clown bordered on suicidal mind you.