Every fish keeper eventually learns to skip the treatment altogether by not buying the disease in the first place. A sick fish brought home can empty a healthy aquarium in a fortnight, and the signs are almost always visible in the shop if you slow down and read them. Treat every purchase like an inspection: of the individual fish, of the tank it shares, and of the shop's habits generally.
Read the fish: fins, skin and body shape
Start with the fish itself. Fins should be spread, intact and held proudly; fins clamped tight against the body signal a fish that is stressed, chilled or ill. Skin should be clean and clear for the species — white spots like scattered salt, a gold or gray dusting, fuzzy patches, red sores or raised scales are all reasons to stop. The belly should be gently rounded, never sunken or knife-edged, and the spine straight. Eyes matter too: bright and clear is healthy, while cloudy, swollen or sunken eyes say the fish has been fighting something. None of these fish is a bargain at any discount; a fish that looks 'a bit off' in the shop rarely recovers in a carry bag.
Read the breathing and the swimming
Watch how the fish moves before you ever ask for the net. Healthy fish hold their level in the water with easy fin movements and breathe at a calm, even rate. Rapid gill movement, gasping at the surface or hanging head-up under the filter outflow all point to gill trouble or poor water. A fish that lists to one side, spirals, rubs itself against gravel and decor, or sits on the bottom while its kind are swimming is telling you the same story. Ask to see the fish fed: an individual that races to the food with its tankmates is a far safer buy than one that watches the flakes fall past its nose.
Read the whole tank, not just the fish
Fish share water, and water shares everything. If any tankmate is dead, dying, spotted or gasping, every fish in that tank has been exposed, including the bright one you want — decline the whole tank, however tempting the individual looks, and come back another week. Check whether the shop's tanks run on a shared filtration system too, because where water is shared across a wall of tanks, one sick tank raises the risk for its neighbors. Clean glass, lively fish in every tank and staff who net carefully are the background signs of a shop whose stock you can trust.
Read the shop's habits, then be patient at home
Good shops hold new arrivals before sale, happily feed a tank on request and answer questions about how long a fish has been in. Be wary of buying on delivery day, when fish are at their most stressed and least readable, and of any pressure to take a fish that is 'probably fine'. At home, a quarantine tank for new arrivals — even a simple one, kept for a few weeks of observation — is the final layer of the same discipline, and it protects the community you already love from the one risk your eyes missed.
Standing at the glass with a nagging doubt is exactly the moment to slow down rather than be hurried by the queue behind you. When the signs still will not settle in your mind, put a follow-up question to KinsLe’s Kinny; he answers using the KinsLe Answers library.