In the absence of fish, the only methods of keeping nitrate and phosphate at desirable levels is either to feed your coral regularly (which contributes more PO4), or to dose nitrate and phosphate.
For more information on dosing phosphates, read this article.
For more information on dosing nitrates, read this article
The addition of fish requires that you feed them, and in doing so you are adding nutrients to your frag tank. Fish food contains phosphate, and when eaten creates ammonia once excreted – which eventually becomes nitrate.
For this reason, I even keep a couple of Clowns in my frag tank. As well as my love of Clownfish, they also help in contributing towards much needed phosphate and nitrate my corals require.