Ammonia in the source water and the low pH can both be a challenge in cycling the tank.
What kind of dechlorinator-does it also cover ammonia-if so, you might be seeing ammonium and not ammonia-as well as the ferts can cause false positives. The beneficial bacteria can't grow in low pH.
I would remove the CO2 and top off the tank with higher pH dechlorinated water to get the pH up some and add an ammonia source to get that increased to at least 3-4ppm for the fishless cycle. Don't touch the filter media unless the water flow is impeded-then only a rinse in old tank water or dechlorinted water.
Since your finial stocking plan is a single Betta...I would get Prime and fish-in cycle with the Betta. And since you have other cycled tanks-swap out filter media with one of the cycled tanks and place it in this tanks filter box to seed it.
Last edited by Oldfishlady; 10-21-2012 at 11:31 AM.