Changing the water too often isn't going to be your problem. I can't even begin to guess what would make them say that. I would still look towards feeding. Cyclopeeze (is that what you're feeding?) is pretty potent stuff and can foul a tank fast. When I feed this (once a week at most) I just shave a paper thin slice off of the end of it. What's in the Fluval? How long has it been there? Do you run any carbon? Test the water you get from your LFS
for nitrates, just to make sure you aren't adding to your problem with the water changes.
Your fish would probably appreciate you changing up their diet a little bit. I feed a rotation of Formula 1, formula 2, mysis, prime reef (all frozen foods) and feed a little cyclopeeze/liquid life more for the corals once a week.
Sand is definitely considered better than crushed coral as it does not hold onto the gunk as well.
Adding to your live rock is going to help tremendously.
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