Sounds like diatoms to me. These are probably due to silicates in the tap water. An RO
unit is a good thing to have...it insures that you get only pure water with no contaminants. A 2 stage usually has a sediment/carbon combo and the RO
membrane. A 3 stage will have separate sediment and carbon filters. It will require less frequent filter changes. The 4 stage incorporates a DI
cartridge. The DI
removes what the RO
membrane misses. It is generally recommended to buy a unit with DI
because it will give you water that is very pure.