I have had my 55 gallon tank for about 4 months now. I purchased it on Craigslist. It came with everything including fish. When i first moved it i used the old sand and my nitrates were very high. Between 80-100. I replaced the sand and most of the water and it dropped to 20. It has hovered there for the entire time with no ill effects I could see on fish or corals. Recently it went to about 40-50 and some of my corals are not opening all the way. My understanding is that nitrates are like credit card debt...no time to get in truoble but a long time to get out. I have been doing 5-6 gallon water changes every day for a week and have cut back on feeding. Yet the nitrates will not go below 40. I am thinking that I will do a twenty gallon change but am worried about the massive change be bad for the inhabitants. Other test are great o amminia, 0 nitrites calcium at 450 phospates 0 ph at 8.3 temp is a constant 76-77. i have a chiller and a heater working in the sump. The heater is controlled by the chiller. I have removed the bio balls and clean the mechanical filters every other day. My 34 tank that has been neglected and has very little rock and 2 damsels is perfect. I use RO water to mix my salt (I use red sea coral pro salt) and one tank is perfect and one is driving my nuts. Am I missing something? Should i do something else? Do i need to do a big massive water change? Any help would be appreciated. Thanks for looking and any help.