I have a 29 gal fresh water tank, I started in Oct 08. I'm running an under gravel filter with a aqua 50 power head, off market 60 wet/dry filter, an skelter250 power filter, what happens is I seem to get a complete cycle and added a few basic fish (mollies) everything going good than my PH suddenly falls off below 7.0, than my ammonia levels go high 4.0ppm and I got sudden death. I changed the water completely started the tank up and again all good readings added fish. All my levels were good did a 10% water change and my PH fell off. 8 PPM ammonia and the tank has been maintaing this level now for 2 weeks no fish. I have readings on nirite of .50 PPM, and nitrate of 5.0 PPM. Question am I in a cycling period or do I need to do something