I don't get this - what's in this tank , kust water, or is there some ammonia or a rotting shrimp or whatever. Ammonia needs to be zero, and that nitrate is a problem, using tap water? Personally I justm ix up the water, heat it and leave it a couple days, then put the live rock in to do the cyccles as is required.
Most phosphate test kits are rubbish. The only one worth getting at a reasonable cost is the Deltec/Merck one, but it will hit you for 35 USD I think. Why do you want one?