The range for cardinal tetras
Keep cardinal tetras at 75 to 78 degrees Fahrenheit, with a pH of 5.0 to 6.2 and low hardness. That red stripe running the full length of the body, nose to tail, evolved in shaded Amazonian blackwater where fallen leaves stain the water tea-colored and acids from decaying plants hold the pH down. Cardinals are far less flexible than the neon tetras they resemble: water that is warm enough but sits at pH 7.4 will keep them alive for a while yet leave them dull, shy, and short-lived.
Why soft and acidic matters for this species
In hard, alkaline water a cardinal's kidneys work constantly against a mineral load its body never met in the wild, and eggs laid in such water rarely develop at all. Soft, acidic conditions also suppress some of the bacteria that thrive at neutral pH, which suits a fish with a modest immune system. The practical lesson is to build the water around the fish rather than asking the fish to adapt: if your tap supply is hard, blending in reverse-osmosis water or collecting clean rainwater where that is legal and safe gives the low-mineral base cardinals need, with leaf litter or peat in the filter nudging acidity gently.
Dialing in the water without shocking the school
Change one thing at a time and change it slowly. Set the heater near 76 degrees and confirm it holds overnight. Lower pH in small steps over days, never in a single afternoon, because a fast pH swing is harder on cardinals than water that is slightly off target but stable. Add new fish only after the tank has run at target for a week, acclimate them by floating and gradually mixing tank water into the bag, and keep the lights dim on arrival day, as wild-caught cardinals in particular arrive stressed from long journeys.
Signs the water is right, and signs it is not
A settled cardinal school glows: the blue line electric, the red deep and unbroken, the group moving as one through the plant shadows. Fading red, fish separating from the school, or repeated losses in the first weeks after purchase almost always trace back to water that is too hard, too alkaline, or too changeable rather than to bad luck. Test temperature and pH before every water change, match the new water to the tank, and treat any reading drifting toward neutral as a prompt to check filters and leaf litter.
Acclimation is where cardinals are won or lost
More cardinals die from a rushed introduction than from any steady tank condition. Shop water is usually warmer, harder, and more alkaline than a proper cardinal tank, so float the bag for temperature, then mix small amounts of tank water into it over the better part of an hour before releasing the fish. Dim the room, skip feeding until the next day, and expect the school to hide at first. A quarantine period in matched soft water, when you can manage it, protects an established school from both disease and the chemistry shock of repeated additions. Buy the whole school from one source at one time where possible, so the group settles together instead of absorbing newcomers every few weeks.
For one cardinal in the school, holding that fish's records and dated notes together in its KinsLe Pet Passport turns checking its history into a two-minute habit instead of guesswork.