In Australia, I learned a technique for deflating bloated swim blatters by taking your fillet knife and making a clean puncture right behind the pectoral fin, under the edge of a row of scales. If necessary, you could give the fish a gentle squeeze to help the excess air out. It sounds brutal, but it works great and reportedly doesn't cause any major damage to the fish when done carefully, usually healing within a few days. The guy who showed me has a doctorate in fisheries biology, so I'm taking his word for it. The technique was developed in Florida to help deal with release of undersized bottomfish such as grouper.