I'm a beginner. A friend gave me a 55 gallon tank. I cleaned the tank, added gravel, artificial plants, and setup 3 air stones. I'm using an Aqua Clear model 70 power filter and an Eheim 300 watt heater. My water temp is steady at 78 degrees. My testing shows 0 ppm Nitrite, PH of 7.8, and 0 ppm...