What is the TDS reading on your RO Unit, and do you test it? How do you know for a fact that the RO water you are putting in your tank is pure and clean to begin with?
What salt mix are you using?
What test kits are you using and how old are they?
How old are your bulbs? I know you told us how long they are on each day, but how long has it been since you changed them last? Also, what brand of bulbs are they?
How much internal flow do you have in your tank? IE the gph coming from all of the powerheads in your tank... Example.... a 20g aquarium needs 400gph minimum for a total of 20x turnover rate. A 75g tank x 20 (turnover rate) = 1500gph of flow from all the powerheads minimum.
It also matters where the powerheads are located, and if you are getting detritus out of your tank.
If you don't have enough sand stirrers and detrivores like nassarius snails, cleaner clams, brittle stars etc. the sandbed can become septic - where nitrate and phosphate clog the sandbed and then it slowly leaches back into the tank.
I do not believe that your nitrates and phosphates are zero. They may be zero because algae is consuming it, but I know for a fact that your tank is no longer handling the bioload that easily.
Do you have a refugium with a light that is changed every three months? What algae do you grow?
Do you have a protein skimmer that you regularly maintain and get at least one cup of skimmate every three to four days?
Have you ever maintained your sandbed with a gravel vac once every six months to a year or have you added stand stirrers and a cup of sand from another established tank at least once a year?
