Personally, I put the fish a gallon freezer ziploc, fill it with fresh tap water, squeeze or suck out as much air as possible(air is the enemy), seal, and lay flat in the deep freeze. Works like a champ. Also, like Dutch said the vacuum sealers are really good, of course it probably depends on the quality of the sealer, plus the bags are kind of pricey. My girlfriend freezes fresh herbs after she vacuum seals them and when thawed they are fantastic.