Hi Rutger,
You acclimatize them like any invertebrate. I used a big bowl for my octopus (Colin's advice) and placed my little octopus and most his water in this large bowl. Then I began to slowly add water from the aquarium, about 1/2 cup at a time. Over a period of about 30-40 minutes, I had more of my tank water in the bowl then the original water. Then I scooped up the little bimac and put him in the tank.
Some people float the bag with the octopus or the fish and add the tank water to the bag. You probably have to remove some water from the bag before doing this. Also, you need to be careful not to pour water from the bag into the tank.
Hope this was the information you were looking for,
Nancy