For a reef tank I wouldnt use any as the LR give plenty of bio-filtration, if its a FO then maybe, thats fine, but some people would fill them up higher.
Take half the bio-balls out for a while, like a week then test your water see were you are at with the nitrates and trites, some surface is good andit hurt to keep half. trial and error keep the fuge it is a big plus :wink: