I used to always fill with tap as well Jeff. Never saw any reason not to. The last few times I've had to reset/refill, however, I've used strictly RO and found an amazing difference in the amount of algae during cycles. With tap I used to get gobs of diatoms, tons of green fuzz and then eventually the hair algae. With using RO water from get go I get just a dusting of diatoms, a faint green fuzz stage and still no hair algae as of yet.