When buying fish, it's a good idea to take your time and watch the fish for a good while to see how they are reacting and behaving with their tank mates. It's well worth remembering that it's far easier to add a fish to your tank than remove it, so be fussy.
Aquarium fish show a very wide range of behaviour and lifestyles but as a rule, choose a fish that is active, bright eyed, well-fed and with good, clear fins. For some species, swimming upside down or sleeping under a piece of wood is normal behaviour!
Beyond that, ask for some advice in your local Maidenhead Aquatics store. Some fish might have small marks, or nicks on fins that might be nothing to worry about, but it's always worth asking. Remember that fish have amazing abilities to change and develop colours. Fish that appear drab or grey in our holding tanks can often explode into vivid colour when moved to a well-furnished home aquarium. Take a look at any display tanks for examples of this.
An old favourite has always been "a hungry fish is a healthy fish". If you are ever in doubt, ask to see the fish feed. Healthy individuals should race to the top and savage whatever food is put in.
As a rule of thumb though, avoid fish with obvious deformities or showing balance or lethargy issues. In the case of new arrivals you'll often see them undergoing a period of observation before going on sale, so let the aquarists in the fish house improve their health and get them fighting fit before you pick them up on your next visit.