I use RO/DI water, and I did test my salt water I use for my water changes and it showed approximately a nitrate value of 5. I use reef crystal salt, so I am assuming the nitrate is coming from the salt but could be wrong. With my system being large I do a 50 gallon water change every week. Have been doing this since October. The nitrates still won't come down. I switched out my bio pellets in October to Tropic Marin hoping that would help but it brought my nitrates down a little and now has leveled off at 20. I don't have an algae problem but I don't think the few SPS I have remaining are the happiest. Growth is slow, color is not great.
I have thought about a algae scrubber but I don't have room between my tank stand and my sump to squeeze it in the sump.