We have the same problem. Tank up for 1 and 1/2 years and all the sudden diatoms all over the sand and some of rocks. We tested and tested and couldn't figure it out. We have been doing 20% water changes about every other day and still nothing til we decided last week that maybe our filters and membrane on our 5 stage rodi may be bad. We ordered those and changed them out last night and we started with a new batch of water mixing this morning and will do a water change tonight to see if the new filters will help. Maybe that is your problem too. Will post an update if it seems to help.