WOAH! STOP!!!!
Your just dumping tap water in your tank and then adding salt and mixing it that way?
When making new saltwater, you should be mixing it with fresh RODI or Distilled water in a separate container than your tank. Many of us use Rubbermaid 20G or so bins to do water changes. If you're using tap (which I really don't recommend) you need to be using some sort of water conditioner and allowing it to stand at least 24hrs before adding to the tank. What conditioner are you using?
As for water changes, 10-20% is the recommended amount.
Reading through some other posts, I'm gathering that this is a new tank? You've thrown a lot of bio load at it in a very short period of time. You have a lot of fish. What do you have for a clean up crew.
What do your parameters look like?
What size tank?
What equipment are you running (powerheads, pump, skimmer, filtration)?
Do you rinse your food before adding it to the tank?
What type of sand?
At this point, I'm not too sure it's diatoms. It's a new tank, so it's very likely, but with the bioload and missing info, I would not rule out dynoflaggelates or cyano. It's probably diatoms, especially since you used tap water.