The sand you added may take months to begin to process nitrate. There aren't any easy solutions to remedy nitrate levels that are as high as reported... particularly if it needs to be done quickly. Water changes are about the only method of rapidly reducing them. However, they must be done in multiple large changes (~50%) and done close together.
Since your Salifert kit is maxed out let's assume the nitrate level is really 200 ppm. That means it will take at least three 50% water changes done back to back, to reduce the nitrate to 25 ppm. If it's higher that 200 ppm, it could take more. If you don't do the water changes back to back, something like every 24 hours or less, their efficiency is reduced.