grabbing air

My husband and I have had our tank for almost a year now. All of our fish and tank mates we've had for about 9 months. 40G Breeder Long with Live Rock 3 powerheads (2 small, 1 big) Ammonia- 0ppm Nitrite- 0ppm Nitrate- 26.9ppm (we know its high, it's due for a water change tomorrow) Salinity-...