I have a 10 gallon tank in which has had fish for almost 3 months now, but my ammonia levels now are extremely high. I've done several water changes and vacuumed the gravel. I have also used Prime to treat the new water, but nothing seems to change the levels of ammonia. My nitrites and nitrates are both at zero, and my ph is it 7.3. Any suggestions as to how to lower the ammonia levels?