I feed my fish a 3"x2" chunk daily (3 of them eat nori), I can't believe the amount of crap it generates on the bottom of the tank. Because of this I have to do water changes weekly to siphon it all up as it forms piles on the bottom of the tank that start to deteriorate as well as look gross. This week I experimented with not feeding nori at all, just frozen as usual and the lack of detritius on the bottom of the tank is incredible. However my fish are getting lumpy guts so its back to the nori I go. In the meantime, they certainly do pick at the rocks more without the nori (eat that damned algae you picky little buggers!!
)