the fish lice aren't so bad unless it goes unnoticed for a while. there is one easy treatment option, but its not so easy to find--remove any visible ones then treat with dimlin or something based on it, its harmless to fish it just makes crustaceans unable to molt properly, so the little fish lice (being crustaceans) wont be able to complete their life cycle and will die out from your tank. or if that was the only one you found keep a sharp eye out for another month or so and if no more appear you should be all set.