Why can I not kick Ammonia?

[digital-audiophile]

I am somewhat baffled as to why I cannot drop the Ammonia level in my new tank.

Current readings

Ammonia : 0.5
Nitrite : 0.0
Nitrate : <30
PH : 7.8

I have had the tank (90G) cylcing for 6 weeks now. I have >100lbs of live rock in there 1/3 cooked rocked, 1/3 rock from my existing tank and another 1/3 of new rock from Red Coral and Elite.

I've been testing the tank every couple of days and these reading have been consistant for about 2 weeks now. I have used two ammonia test kits to make sure I was not getting a false reading. My PH is low too but I have started dosing with a PH buffer to help bring it up.

It just seems odd to me that I continue to test 0 Nitrites and high nitrates but still have trace Ammonia?