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Acclimating fish is much simpler than most people make it out to be. Over the years of acclimating hundreds of my own fish and thousands in the stores I have worked in and run I have learned that acclimating is actually a much faster and simpler process than most people are led to believe.
For me there are two types of acclimation, the standard (quick and easy) and the drip acclimation. In almost all cases (including saltwater fish, inverts, and corals) the standard method is the better option. In a few cases the drip acclimation is necessary.
For the standard acclimation float the bag in the tank. After about 5 minutes open the top and scoop in a few small scoops of water with either the bag itself or a small cup. If you can untie the bag to do this do so, otherwise tear or cut a small hole in the top of the bag as close to the knot as you can. Do this every 5-10 minutes until the bag is completely full of water. This should take 20-45 minutes depending on the fish and how well they handle the acclimation. If the fish are active, alert, and look like they are wanting to swim around you can finish the acclimation in as little as 20 minutes. If they sit on the bottom of the bag, breathe heavily, clamp their fins, or show other signs of stress keep it slow, don't rush them.
Don't hesitate to take a little more time if you aren't sure. It is better to go longer if you aren't sure they are handling it really well. Watch the fish, they will tell you how they are handling it. Fish like zebra danios, platies, barbs, etc. should be pretty fast. The water quality inside the bags can get pretty bad so the faster you can properly acclimate them the better off they will be.
First, it shouldn't be an actual drip acclimation so I don't like this name, but that is what everyone calls it so that's what we'll go with. The usual drip method is to pour the fish in a bucket, start a siphon with an air tube, and tie a loose knot in it to slow the flow down to one to a few drops per second. The problem with an actual drip acclimation is that it is too slow. A drip means the water is going through the tube so slowly that it cools off. You are basically acclimating the fish to room temperature. In addition the water in the bucket cools as well, again acclimating the fish to room temp.
A better method is to use an air tube like the usual drip method but to actually leave it completely open. This doesn't allow the water in the tube or bucket to cool. This goes much faster but because it is just an air tube siphon the amount of change at any one time is minimal, especially compared to doing scoops of water at 5-10 minute intervals. You basically get the same acclimation time as a standard acclimation but the water is added at a constant, slow rate. Because of the rate of the siphon you need to keep a close eye on the bucket. It seems slow enough but if you leave it alone you risk flooding the bucket.
The drip acclimation is really only necessary in cases of sensitive fish (or very expensive fish). I would consider the drip method for fish like discus, fancy plecoes, wild caught fish, freshwater stingrays, maybe something like cardinal tetras which can be a little more sensitive, etc. Most saltwater fish are fine with a standard acclimation if you watch them and go longer if needed but some will do better with a drip acclimation.
Releasing the Fish:
The actual release of the fish is pretty simple and straight forward. Once the acclimation is complete you can either pour the whole bag into the tank or pour out most of the water in the bag into a bucket leaving just a minimal amount of water in the bag with the fish in it to pour the fish into the tank. I know this flies in the face of everything other people like to say. They are worried about introducing waste in the water or pathogens. The truth is that we don't dry off and sterilize our fish so if some pathogen is coming in, it's coming in either way. If the water quality was really bad then yes, you may want to pour out most of the water before dumping the fish in the tank. In most cases the waste that is built up in the water is practically nothing compared to the whole volume of the tank and the bacteria will eat it up no time, but if the fish have been in the bag a particularly long time or were a little overcrowded by the person at the store who bagged them then you will not want to add the water to the tank.
With the drip method you can simply net the fish out of the bucket.
If you can gently grab the fish with your hands this is a little easier on the fish, but if you have to chase them to get them you undo that benefit. This is easiest with fish like fancy goldfish but sometimes discus are easy to grab too (sometimes they are too jumpy and fast so don't push it).
You can also buy the same plastic containers that hang on the tank that the fish stores use when they catch your fish. These can be great for any aquarist to have around. If you gently corral the fish into the container using a net moving them with the container is better than a net. You can pour out extra water from the container.
Feed After Releasing the Fish:
Think of your tank as a prison. If you were in prison but it wasn't too crowded and minutes after you get there a buffet opens up it is still prison, but not too bad. The same applies to fish. If you feed right after they are released the feeding will distract the old fish from the fact that there are new fish in the tank. The new fish will at least get a chance to settle in without harassment and ideally they will actually eat.
Lights On or Off?
Some people like to turn off the lights in order to reduce stress. This can help but isn't usually necessary except in cases of skittish fish. Any nocturnal fish, discus, jumpy tetras, freshwater stingrays, and other similar fish should have the lights turned off for the rest of the day. If you would like to turn the lights off for all fish that's fine, but active, hardy fish will be happy to settle in without issues.
I worked at a store that for some reason thought it was less stress to leave the lights on all night after we got new fish in. This is about the dumbest thing they could have done, and not the only moronic decision they made. Don't do this. And this is a reminder that just because your local store recommends certain things doesn't mean they are right.
Rearrange the Decorations?
In some cases rearranging the decorations can be helpful. This is really only necessary with territorial cichlids where the established fish will be very aggressive toward new additions. So while you are acclimating the new fish you will want to rearrange the decor in order to disrupt their territories. In these cases I would feed right after releasing the new fish and cut the lights. You need to use every trick in the book to prevent any issues.
Xenia Coral Acclimation:
One major exception to both methods is Xenia coral. I would do the same process as the standard method but MUCH slower. It should take at least 45 minutes if not at least an hour for a nice slow acclimation. Again you can watch. If Xenia is acclimating well it will start to very slowly pump. If it stays tightened up like a fist give it a little more time. For some reason Xenia is really bad about acclimating to new tanks and it is not uncommon for a recently added colony or frag to completely melt within a few days so take it slow.