I am new to having an aquarium. I've spent a lot of time reading up on the nitrogen cycle and understanding how to care for fish. However, I am not prepared on how to handle this situation, and am at a loss for what to do. 1. 10 days ago a friend was moving, and passed on their aquarium to me. It was drained, rocks were not rinsed off but everything else was. Tank was well established and had no problems. The tank was re-set up in my house, and about 25% of the water was transferred with the fish. Everything was treated and put back together. 2. 5 days later everything seemed to be fine, and then the filter died. I went out and got a new AquaClear 50 filter (20 gallon tank) as I had read good things. I did a water change and gravel vac when I replaced the filter as it had been off for a few hours so I thought a 30% change might be good. I also purchased a new fish as there had been a previous death a few months back and I thought I'd replace him. I also added some nutrafin cycle as it was recommended by the store. 3. Here we are today: I use the API test kit and test everything again, previously everything was 0 and the pH was 7.0. Today, everything was the same but the nitrates read between 10-20. I did a 30% water change and gravel vac, and re-tested the nitrates about 15 minutes later, and got a level of 80ppm. Yes, I did followed the directions and shook everything exactly as directed. What gives? What do I do? Is my tank in some sort of mini-cycle? Am I overdoing it? Under doing it? Too many fish? (4 glo tetras, 3 neon tetras, 2 mollies, and 2 other fish I don't know the size of the mollies, and 1 pleco that has grown to about 6 inches) Do I need to get some Nitra Zorb to put in the filter? Previously the tank had been under monthly maintenance only. Thank you in advance! Oh--forgot to mention I am using Prime to treat the water, and Tetra tabs to control the pH as we have really hard water.