I want to add additional fish to my freshwater aquarium, and I want to make sure they have the least chance of dying. After considering the answers to this question about water, what else should I take into consideration?
So... What I would consider/do is:
Tank capacity according to the fish you already have and the size of your current tank. Bear in mind that these are rules of thumb.
Feed the fish in your current tank before adding new ones. That will help with any potential aggression as that is commonly related to food sources.
Make sure that there's plenty of places for the newbies to hide. May be needed initially if there are dominant fish in the tank.
You probably don't add the total planned all at once. Do it in stages and let your water quality stabilize before adding more.
To add some more tips I do in my aquariums:
Remember to add some more food after you have inserted those new fishes. It's easy to forget and keep with the same amount, what might stress your older fishes (since there will be new mouths disputing with them)
Take care with old /weak fishes: if your fishes are a bit old or aren't in good health (not just sick, but debilitated somehow) they can be hunted by those new fishes, if they are disputing territory, food, a place in the sun...
I'd put them in the night, with few light or light off, except if some of the old/new are nightly fish
Add some plants (if you don't have many) so that the aquarium changes slightly, and not only the new fishes are the news there.
In addition to making sure the new fish are a good fit for your tank, you'll want to prepare for any diseases or parasites they might come in with. These are often latent and brought out by the stress of moving, so you can't rely entirely on picking only the healthiest fish in the tank. (Though of course that's a good thing to do.)
Ideally, set up a small temporary quarantine tank to keep new fish in before adding them to your community. Watch them closely for 2-3 weeks for any obvious symptoms like white spots or fuzz, really long trailing poops, or red streaks, and more subtle things such as loss of color, weird behavior like flashing against the gravel or swimming off-balance. Some fish are known for having particular ailments (like neon tetra disease), so it's a good idea to read up on those in advance. And if you can't QT the new fish, then you'll need to watch everyone in the tank, not just them.
Fish disease would be a bit off-topic to really go into here, but just be aware that there are different classes of disease that require different treatments, just like with humans. There are no one-stop-cures despite what you'll read on product packaging, so diagnosis is the first step -- that can be very hard and unfortunately in some cases time-sensitive, so don't hesitate to ask for help if something looks out of place after you've added new fish.
QT and all this is a pain and not terribly fun to deal with, but believe me when I say there is nothing more heartbreaking than seeing a whole tank community come down with the plague. Thinking about it when you're in the planning stages is so much better in the long run.
None of the answers addresses two of the most important considerations.
- Quarantine your new fish after purchasing, for at least 7-15 days. This makes sure the new fish does not bring anything bad to your tank.
- Acclimate them by matching the water temperature, pH, TDS of their old home and the new one. This is usually achieved by adding your tank water to the fish bag repetitively by a small amount.
In addition, make sure the new fish is compatible to your existing ones. It might be necessary to change the decorations slightly to break any territorial aggressions. I usually keep the light off after releasing them into tank and let them settle in. Do not try to feed them immediately, they will not eat and the water will get foul. Do not add more than a few fishes at once, otherwise the tank cycle might restart.