i'd say do a 50% change, and its not specifically the refug that changes around that nitrate, but the plants. i have caulpera in my tank to do some exchanging, but i mainly like it for its color. if you don't like caulpera (its a little aggressive), find some cheato, but that usually does best in a refug. 17 is really high, something else is going on in that tank, it can't just be from a high load...anything die (or wen't missing) lately, or within the last few months?