Unfortunately, it's quite common for fish to die shortly after being brought to their new home from the pet store. While many new fish owners are quick to blame the pet store for selling fish in poor health, in reality, this untimely passing is often the result of budding aquarists not knowing how to properly acclimate aquarium fish to their home aquariums.
Most people just float the bag in the tank to equalize the temperature and then dump the fish, water, and all, into their tanks. Not only is this a poor way to acclimatize fish, but dumping pet store water into your tank is a bad idea. Water from the pet store can contain diseases and parasites that will be transferred to your beautiful community tank at home.
All new fish being put into a community tank should be quarantined for two weeks, at least. If you don't have extra tanks to quarantine in, you must be very observant of the condition of the fish dealer's tanks. If there are sick fish, dead fish, or ich on any of the fish in the dealer's tanks, do not buy fish there and put them in your community tank. They must be quarantined to protect your little beauties at home.
Open the Bag
When you get your new fish home, turn down the lights to avoid shocking your new pet. Your next step should be to take off the rubber band and open the bag. Place the bag in the tank, so the water supports it. Next, roll the open top of the bag down four or five turns, so it creates a ring of air trapped in the rolls of the plastic bag. Now the bag will float on its own without tipping over. If it is still unstable, a couple more rolls might be needed.
Test and Tweak pH Levels
More new fish have been killed by pH shock than from any other problem after adding them to a tank. Differences in pH, while they may not sound significant, can be lethal to a fish. In fact, as little as a 0.5 difference can send your fish into pH shock. This is something they may recover from, or may not, depending on the severity of the difference in pHs. The bigger the difference, the more chance of your new fish dying from it.
To avoid pH shock, while the temperature is equalizing, you need to test the pH of the water in the bag and compare it to the pH of your home tank water.
If the two pH's are very close (within two-tenths), the two will equalize in about an hour. If the pH's are off by more than 0.5 up to 1 or even 1.5, this will take longer. Remember, you are giving your fish the best chance of survival in your tank. You could even drop a few crystals of ammo-lock into the bag to absorb any excess ammonia if it was a long trip from the fish store.
Add Water From Your Tank to Your New Fish's Bag
Acclimating is a slow, steady process. To begin, dip 1/2 cup of tank water from the tank and add it to the bag. Now wait 15 minutes and do it again. This slow process will allow your new pet to acclimatize to a change in pH as well as new nutrient levels, oxygen content, salinity, sounds, and lighting.
With a 0.8 difference in pH, you will need to do this every 15 minutes for at least two hours. That is eight times before you net them out and release them in the community tank. This assures that the water is very slowly changed from their bag pH to your tank pH without any abrupt shock to their little fishy systems.
With a difference of less than 0.4 you can drop the time to one hour, but with a larger difference, like 1.0 or larger, you will need to increase the time to 3 hours or longer and maybe even use a 1/4 cup measuring device to add less water each time for a longer period. While there is no absolute rule of thumb regarding this process, you may need to take up to four hours acclimating a very expensive, delicate fish to your tank. Discus are a prime example of a fish that needs a long acclimatizing period even though the bag water may be close to your tank water.
Temperature Versus pH
A slow, careful acclimatization process will give your new fish the best chance for survival in your tank, although it will not protect your other fish from any diseases or parasites that the new fish may be carrying.
Remember, the temperature is the least of your worries when acclimatizing a new fish. Also remember, your new fish's life is in your hands, and it depends on you to make the right decisions.