Freezer is cold but not freezing at all
15-20 degrees isn't cold enough, believe it or not :)  There needs to be a significant temperature difference to rapidly draw the heat out of the items you want to freeze.

For example, current Whirlpool refrigerator/freezer units come preset to 0-degrees (F, not C) for the freezer and 37-degrees (F) for the fridge.  According to this document, the freezer range is from -5 to 5 (F), and the fridge can be set from 33 to 41 (F).

Make it colder, and save your fish :)