Diatoms are not a threat to fish. They are a single celled algae that is used for food by many animals including fish, snails, clams, etc. They are just unsightly when there's an overabundance.
The best way to restrict Diatom blooms is by minimizing the amount of Silicate you add to your tank. Silicate is the basic building block for Diatoms and are available by the truckload in tap water. RO typically removes 70% of Silicate from tap water, DI will remove about 97%. The Hi-S RO membranes will remove about 95%.