As I'm sure you read, crushed coral is tough. It collects alot of waste, and has to be vacuumed every week with water changes. Do take the advice and add a powerhead. The out from your filter is good, but not near enough for a tank that size. Consider the puffers natural habitat and all the current running through it. The added flow will be very helpful in keeping all that shrimp dibris in your water column and out of that CC. Dead spots, areas lacking flow, are great areas to promote decay of uneaten food.
Also, let us know a little about your skimmer. It should be helping you, lets find out why not!
Also, let us know a little about your skimmer. It should be helping you, lets find out why not!