If you have 15 ppm nitrate, a 25% water change will only bring your nitrates down by about 4 ppm, leaving you with 11 ppm nitrate.
If this was my tank, I would do a series of 30% water changes over the course of a week to bring nitrates closer to 0 pmm. Then, every week I would do at least 15% water changes. If there is any mechanical filtration in this tank, I would also thoroughly clean the filter media in the weekly outgoing changewater. Doing so will remove any crud buildup, which is a source of nitrate and phosphate.
I would also test for alkalinity, calcium and magnesium every other week and balance these three important reef chemistry components. Here are some links explaining reef chemistry and how to find the right balance for your tank:
Aquarium System Volume Calculator:
http://home.comcast.net/~jdieck1/volcalc.html
The Reef Chemistry Calculator:
http://home.comcast.net/~jdieck1/chem_calc3.html
Reef Aquarium Water Parameters:
http://reefkeeping.com/issues/2004-05/rhf/index.htm
Magnesium in Reef Aquaria:
http://www.advancedaquarist.com/issues/oct2003/chem.htm
Solving Calcium and Alkalinity Problems:
http://www.advancedaquarist.com/issues/nov2002/chem.htm