50% water change and still 20ppm nitrates

[Milad]

yes im using RO water

No i didnt test nitrates all the time, i just tested after i saw the oil slick

i dont have a phosphate test kit so i havent tested it yet.

the rock i believe is decently old, i bought it from another reefer (mikeolsen) i believe he had it for several years.

this all started when the oil slick on the top of the tank showed up. The changes I did during the oil slick time was remove rowphos and start zeobak, coral snow, sponge power, zeo zyme, cv.

Then i stoped most of the zeo stuff and just did zeo zyme and cv and bak to try to get rid of some cayno (which is still there).