The ideal storage temperature for seafood is 20 degrees below zero. Even if the temperature fluctuates between 20 below and zero there will still be damage done to the product. This slight flexing of the cell wall will make the fillet mushy. Most of the damage is done during shipping or if the product is located near a freezer door that is frequently opened and closed, allowing the temperature to go up and down. Signs of temperature fluctuations are ice crystals, distorted shape, clumping and discoloration.
Fish fillets are 70-80% water, which makes them a very delicate product to handle. To be properly frozen, the temperature must drop from 39 degrees to zero almost instantly to keep the water from expanding. If it expands, the cells are damaged, and the fillet becomes "mushy." If the product was properly frozen and it is still mushy, the problem is most likely in storage temperatures. If the temperature fluctuates from -20 to zero degrees, even this will cause a slight "flexing" of the cells and will cause damage. The ideal storage temperature is a consistent -20 degrees.