Acclimating new fish to your water
Acclimating fish. How you treat new fish in their first hours can make a difference in their survival rate. The most important advice I can give here: Don't add the traveling water to your home aquarium. After you've acclimated new fish to your water, gently net them out and discard the travelling water, along with its load of ammonia and perhaps pathogens.
Of course you'd never just net a fish and drop it into a new tank. But I've spent many clumsy hours fiddling with plastic travel bags, and I've found that when you're transfering fish from the LFS or even from one aquarium to another that has markedly different water characteristics, it's always less stressful to use the drip method: Empty the travel bag into a clean dark aquarium-use-only bucket. Set the bucket on a sturdy stepladder or kitchen chair next to the new aquarium, with a length of air hose leading from the aquarium to the bucket. Tie a knot loosely in the air hose, or use a plastic in-line valve, and adjust it to let water from the new aquarium drip slowly into the bucket. Don't get side-tracked and let the bucket overflow. And keep the bucket covered: fish are quieter in the dark. Net the fish out of the bucket into the new aquarium and discard the water. Finally, top up the aquarium with fresh water.
If you don't have the patience for this drip method, and you're floating the travelling bag in your aquarium, at least roll out the bag edges and clip one side to the upper frame of your tank with a plastic clothespin. This avoids the bag tipping over and spilling travelling water and fish, or closing up when you turn your back and stressing the new fish from lack of oxygen.
The transport "breather" bags that permit exchange of oxygen and carbon dioxide and are completely filled with water, with no air space, cannot be floated in the aquarium without risk of suffocating the fish. Here the drill is to empty the fish in its traveling water gently into a small bucket and begin your drip method.
If you use the drip-bucket technique, then you're already set up for a prophylactic treatment against skin and gill flukes (monogenetic trematodes). Prepare a brine of common table salt (not sea salt; you don't want to boost the buffer or the pH just now), using 5 level tablespoons per gallon of water in the acclimating bucket, which you premix by stirring it into a cupful of tank water. Add half the salt brine, wait fifteen minutes, then add the rest. Keep a close eye on the fish and net them out at the first sign of distress: gasping and gill pumping, dashing or staggering motions. After a half-hour, net the fish out and into the aquarium. Salt is less toxic than formalin, which you might use instead in this technique. Praziquantel is a new, less toxic alternative.