I think I have read that the ammonia tied up when you use Prime will still show on an ammonia test. If you are double dosing you shouldn't need to be concerned at all. You seem to be using Prime much faster than I do. I dose a 45 gallon and ten gallon tank, at water changes, per the bottle instructions, and it lasts close to a year for me - same 250 ml bottle. Each water change on the 45 gallon tank takes only 5 ml, and on the 10 gallon only 1 ml.