Your post says that when you initially filled the tank you used "regular water with the Prime stuff." That is where a some of your problem is coming from. The diatoms need silicates to make thier shells, and regular tap water is rich in silicates. Once the diatoms use the available silicates they will die off, however if you replenish your tank with tap water that has not been filtered through an RO/DI you will get another bloom.

Welcome and it sounds like you are well on your way to having a nice saltwater tank.

Bill