Ask where the cardinal tetras came from
Ask whether a shop's cardinal tetras are wild-caught or tank-bred, and how long ago the group arrived. Most cardinal tetras in the trade are collected from soft, dark South American waters, and newly imported fish need time to recover from transport before they are moved again. A group that has rested and fed in the shop for a while, holding its full red and blue colour, is a much safer purchase than fish still pale from shipping. Ask which system the cardinals are kept on, and look at the whole tank for flashing, spots, or fish hanging apart from the school, since schooling fish share each other's fortunes closely.
Ask how big a school to buy
Ask how many cardinal tetras the shop recommends for your tank, and expect an answer that pushes you toward a generous school rather than a token few. Cardinals are true schooling fish whose colour and confidence build with numbers, and a small group in a large tank often hides and washes out. Ask whether the shop will hold a full school for you if its current stock is low, instead of selling you a handful today and strangers later. Ask about the adult size the school will reach so you can judge your stocking honestly. A shop that talks about the school's long-term behaviour, not just today's sale, is worth returning to.
Ask about the water in the cardinal tank
Ask what temperature, softness, and acidity the shop keeps its cardinal tetras in right now. This species comes from warm, very soft, acidic water and is famously unforgiving of hard, alkaline conditions and sudden swings, so the gap between the shop's water and yours shapes how the fish will cope. Ask how the shop softens or prepares its water if your local supply is hard, and whether it sells the fish with any acclimation guidance. Ask how stable the system is, because cardinals kept in a fluctuating shop system arrive home already stressed. Matching conditions closely for the first weeks gives the school its best start.
Ask what the school is feeding on
Ask what the cardinal tetras are eating in the shop and whether you can watch a feeding. A settled school darts for fine foods as a group, and that response is one of the clearest health signals this species gives. Cardinals have small mouths, so ask about food size, and whether the group takes prepared foods or needs frozen and live items to hold condition. Ask how recently the group was last fed, since fish that were just fed heavily may look quiet for innocent reasons. Note the exact foods named so your home feeding continues the same pattern while the school settles.
Ask about losses, holding, and aftercare
Ask directly whether the shop has lost cardinal tetras from this batch, and listen for a straight answer rather than a sales line. Ask if you can reserve a school and collect it after a settling period, whether the shop quarantines new arrivals, and what it advises if a fish fades at home. Shops that keep good records of their livestock usually keep good livestock. If you are still comparing where to buy, KinsLe's Pet Services Directory lets you find trusted local pet businesses and connect with a fish shop whose cardinal tetra advice you can test with these questions.