Start With the Shop's Tank, Not the Fish
Before you look at any single fish, judge the tank it lives in. The glass should be reasonably clean, the water clear rather than cloudy or yellow, and the fish in the tank spread through the water instead of all crowding one corner or hovering at the surface. A dead fish lying unnoticed, several fish gasping at the top, or heavy algae on every surface tells you the tank is not being watched closely, and fish from an unwatched tank carry whatever that tank has been incubating. One sick tank does not condemn a whole shop, but it does rule out buying from that tank on that day.
Read the Individual Fish Closely
Once a tank passes, study the fish you actually want. Healthy eyes are clear and bright, not cloudy, sunken or bulging. Fins should be held open and intact; fins clamped tight against the body, or ragged and split, point to stress or fin rot. Look along the body for white spots like grains of salt, fuzzy patches, red streaks or a belly that is either pinched hollow or swollen. Watch how the fish swims: steady and level, exploring, is what you want. Flashing against gravel, shimmying in place, or hanging motionless under the filter output are all reasons to leave that fish behind.
Ask to See Them Eat
Feeding time is the most honest test a shop can offer. Ask a staff member to drop a little food in the tank. Healthy fish of most species rise to it quickly and compete; a fish that ignores food, spits it out repeatedly, or has to be almost touched by it before reacting is telling you something is wrong that its appearance has not shown yet. If the staff will not feed the tank, that is information too. Strong shops are used to the request and treat it as a sign of a careful keeper.
Questions Worth Asking Before You Pay
Ask how long the fish have been in the shop. Stock that arrived that morning is still stressed from shipping, and many good keepers prefer fish that have been in the tank a week or more, eating and settled. Ask what the fish are being fed, so you can offer the same food at home while they settle. Ask whether the shop quarantines new arrivals, and what the water in the tank tests at if you are buying for a tank with particular needs. None of these questions is awkward; how willingly they are answered tells you how the shop treats its livestock.
Bring Them Home Well
Choosing well continues past the till. If you can, keep a small quarantine tank at home so new fish spend their first couple of weeks under observation rather than introducing anything to your main aquarium. Float the bag to equalise temperature, then add the fish without tipping the shop water in. A healthy fish, chosen patiently, still deserves a gentle first fortnight. Keeping a note of where each fish came from and when helps you spot patterns later, and the local shops worth returning to are easy to keep track of when you look them up through the KinsLe Pet Services Directory and stick with the ones whose tanks passed your inspection.