Two main aspects of selecting livestock for your aquarium come into play.
First off you would ideally make a selection of fish/invertebrates/corals and cross check and research them to ensure they are compatible with each other and be within your abilities to keep happy & healthy.
Once you have a finalised wish list of livestock and your aquarium is ready for supporting life the second step is selecting livestock for purchase at your LFS.
Some things to consider when selecting livestock are ethics and sustainability, health or behaviour and the cost.Ethics & Sustainability
Many of the fish, corals and invertebrates we keep are sourced from wild locations, but there is lots of work going on and lots of practices already in place that almost entirely and in some cases completely remove any impact on natural sea life.
There are strict laws in place to maintain the sustainability of Australian wild caught specimens and these laws are well regulated and enforced, wild caught or collected specimens are very important to the hobby and the export market, done correctly there is nearly zero impact to the environment.
The same cannot be said for wild fish collection in other parts of the world so buying Australian fish is always a plus.
Also with Australian fish they also have shorter exposuer to stress by going from collection to wholesaler to transit retailer to you.
Imports will will generally go from collection to wholesale/exporter transit importer/wholesaler to transit to retailer to you and a longer more stressful time for marinelife.
In recent times many companies that wholesale supply or retail marine live stock are actively engaging in breeding certain fish species, ornamental clams, shrimp and the propagation of live corals known as "Frags".
If you want to make an even more sustainable choice you could consider one of those options.
Health & Behaviour
Taking the time to observe before purchasing a fish is a good way to maximise the livestocks chance at a happy life in your aquarium.
Obvious alarm bells are lots of dead fish or diseased corals in the sellers system but deaths do occur in holding so have a chat with the seller and look into it further with them.
Things you could ask the seller.
Things to look for that indicate poor condition of the live stock:
When dealing with live animals that are going to be totally in your care, this aspect shouldn't matter too much but some things must be noted.
More expensive doesn't mean better and cheap doesn't always mean nasty.
There are so many factors that set the price for livestock so this is an area where you'll need to build a good relationship with your LFS and trust that they are giving you a good price for the best quality they can give you.
Look after them and they will look after you. Do your homework and ask around the Reefing community for where the best quality is coming from.
Information provided by:
MARINE AQUARIUM FANATICS AUSTRALIA