Several factors might be the cause of your diatoms. For starters how is nitrates and phosphates? Increased nitrates and phosphates cause diatom outbreaks. do you have any cleanup crews? Trochus, astrea, and nassarius snails do a great job of filtering sand. I'd start with that over sand sifting star fish. I also recommend a dragon goby. Mine spends all day sifting sand. Also get some copepods and add then to your setup. They do a great job of eating waste and fish love them. Verify that you don't have any dead spots in your tank. Circulation plays a huge part in good water quality. Once you used tap water then it will take several water changes to filter out the impurities. I had a 55 gallon setup that I used tap water in and never could get my diatoms under control. When I upgraded I only used RO/DI water and after my new tank settled in I have no diatom issues at all. I also have 0 nitrates. Make sure you have plenty of space around your live rock for good circulation. Also do not over feed. I feed my fish every other day if that. Once you establish coraline algae on your life rocks the fish will nip on that and your pods in between feedings. I have pods in my fuge that are 1 cm in length and thousands in my main tank that I see at night when I shine a flashlight. Hope this helps.