While the Marine fish are in their bags they are safely packed with oxygen, so there is no need to immediately cut the bags open. This is the time you can start getting the fish acclimatised to their new environment.
MATCH THE TEMPERATURE TO YOUR AQUARIUM
Start by trying to get the temperature of the tank and the temperature of the bag the same. One easy way to do this is by simply floating the bag in the tank. No need to open the bag, just let it float in the tank for around 15-20 with lights off.
MATCHING THE WATER CHEMISTRY
Once you have the temperature sorted, it’s time to match the water chemistry. The water in the bag will be different to what is in your aquarium. The easiest way to do this is to ad aquarium water to the water your fish is already swimming in. There are many ways to do this. Some will cut the bag open and ad aquarium water to the bag. Others will put the fish into a bucket with the bag water and ad some aquarium water to the bucket. Either will do the job. Once the fish are swimming around in a few cm or so of water ad water from the tank to the container/bag. Until you have around 2-3 times the volume of what you started with. Let them sit for a short period of time to fully adjust to acclimated water perimeters.
RELEASING YOUR FISH TO YOUR AQUARIUM
This doesn’t have to be over complicated. You have taken the time getting them acclimatised to the water they are about to be released into. But a gentle release into the aquarium would be preferred by your fish. Start by turning off the aquarium light. This will help keep the new fish calm when they first enter the aquarium. If Your aquarium has fish swimming in it already, turning off the light will also help prevent the existing fish thinking you’re about to feed them. (You don’t want the new fish being mistaken for food by the old fish). The final step is the release. You can gently tip them water and all into the tank. Or scoop them up with a fine net. Or even better if you have a small cup/container that can scoop both the fish and some water with the fish and release them that way it will leave the fish in much better condition.