Hello Andrew,
Are you sure your reading the test correctly? I looked at the Aquarium Systems website and they say the range the fasTest copper test kit is 0 - .25 mg/l. Also you said you had used coppersafe which is a chelated copper and I think the fasTest kit is designed for testing ionic copper like Seacure from Aquarium Systems. I would not expect you to have high copper levels from a filter if you only used coppersafe a once or even a few times. In any event just use carbon or polyfilter and you should not have any worries. I actually treated my main tank (with 140 lbs of liverock in it) with copper twice to get rid of an ich problem. It knocked back some of the life on the rock but everthing grew back after about 6 weeks. I have since added corals with no problems.

Richard
Cb Pets