Not enough stamps to exchange for Odin Fish? Not to worry, because Happy Fish is extending the exchange period for Odin Fish up to May 31, 2014 (got this information from the Taiwan-version Happy Fish).
Happy Fish normally extends the stamp exchange period for two months if the stamps required for the fish exceed thirty, probably Happy Fish forgot to add in that information in this month's stamp exchange page.
Anyway, learn to keep more stamps by visiting this page - Daily Checkin' Stamps.