1) When packing your fish freeze then up slightly before vacuming. Reason for this is you suck up the juices through the end of the bag and you will seal it with juice in your sealing area. You will find that your seal will fail more often then if it is clean and dry were you are sealing. If filleting your fish they lay flat and stack easier.
2) If doing fillets put something over the bones in the upper half of the fillet or they will poke throught your bag causing failure( folded paper towel).