Most likely the source is your tap water. I had same problems until I switched to rodi
water. Google red slime algae and there is some good info as to what they need to grow an thrive. Also, some sand will give problems for a long time if there were a lot of silicates in it. I would start with water quality first, and go from there. I don't have time now to go through all the things to rid it, that's why I mentioned the google part. Hope it helps.