We have a 75 gallon SW. It has been up and running for about 3 months. We have a 20 gallon sump, protein skimmer rated up to 300 gallons, levels are good, ammonia 0, nitrate 0, nitrite 0 and ph 8.3. We only use RO/DI water. With weekly 15% water changes and cleaning of the sand. But we are still...