The first thing you need to do is add another filter! You should always have two or three on any size tank and yours is rated up to 150g which means you need to double that! You could even add a hang on filter. I would recommend buying one larger than 6" and housing him with only a few tank mates. Also as stated above 25-50% water change. If you don't have a python or equivalent to do WC than you need 1! Find out what the Arow is eating when you buy it and if you are having trouble eating than try live crickets.