There are a couple of keys to remember with heat packs.
First, they need air to work. I usually use a styrofoam box inside a cardboard box. I don’t tape the styro and when I tape the cardboard I only tape it so that it stays closed. I don’t tape all the seams because we need to get some air exchange into the box. If the heat pack doesn’t have enough air, it will stop working. There’s nothing worse than receiving a cold box only to have the heat pack reactivate and heat up after the box is opened. This also affects the number of heat packs to use. More is frequently not better. In small boxes, one heat pack is sufficient. If you use too many, they may run out of air and stop working. If you’re shipping large boxes, like those that wholesalers or exporters use, you may need to use additional heat packs.
Second, if the heat pack gets wet it won’t work. While the logic may be to put the heat pack on the bottom of the box because heat rises, this is not a good idea. It should be in the top of the box to decrease the chance that it gets wet and also because there is likely to be more space and hence more air at the top of the box.
I like to use a couple of pages from a newspaper to line the box and wrap over the fish and then place the heat pack on top of the newspaper. Many heat packs have an adnesive on one side that will allow you to attach them to the top of the box. That will keep them in place. Alternatively, you can tape them to the inside of the box lid. If you’re taping them to the lid, just tape around the edges. If you tape across the actual heat pack you will limit the airflow and you don’t want to do that. Alternatively, you can wrap the heat pack in a piece of newspaper and lay it on top of the fish bags.
Mark