Most flat rate boxes from the post office are only 6 inches tall.Too short to pack your bagsof fish upright,so I lay mine down on a slight angle.I cut out a piece of styrofoam and tape it to one side of the box(Picture). Then I lay my bag of fish on a angle,so when the box is lying flat most of the water will go to one side of the box.(picture).The area right under the bag is where I place my heat or cool pad,remember to tape everything in place.You never know what is going to happen to your box !!Make sure you also tape your bags of fish in place,fill the remaining space with paper or peanuts(Not the kind of peanuts you eat).Close the box,tape it ,and fill out the information on the top of the box,(picture)or if you are using a regular box fill out the label from the post office(picture)remember don't put the label on, let the clerk at the post office apply the label.
Have been shipping for 30 years now.When I started shipping I would write Tropical Fish Fragil,on every box.A couple of years ago the guppy club,which I'm a member of, had a sit down chat on shipping in which they recommend, that only Perishable should be written on the box.I now write nothing on the box. When you bring your box of fish to the post office they will weigh the box and they will ask you if there is anything hazardous in the box and if there is any liquid in the box,you reply yes water,they are going to as if you packed the box so the water will not leak out? You will reply yes,everything is double rapped,in a insulated box.It is not illegal to ship Tropical Fish,so be truthful with your answers. I know that I probably forgot a few things,so if you have any questions feel free to ask them.I only hope this was helpful to you.