The range for Congo tetras
Keep Congo tetras at 76 to 81 degrees Fahrenheit and pH 6.0 to 7.0. From the middle Congo River in Africa, they are a true river fish: warm water, gentle current, and chemistry on the softly acidic side. A male in the right water is a showpiece, with violet and gold flanks and long, trailing fin edges; the same fish kept cool or in alkaline water stays grey-brown and short-finned, and many keepers never learn what they actually bought.
Space and water work together
Congo tetras grow to roughly three inches and swim hard, so their water numbers cannot be separated from tank size and filtration. A school in a long tank with strong filtration holds temperature evenly and dilutes waste between changes, while a crowded small tank swings in both temperature and pH and shows it first in frayed male fins. Slightly acidic water also keeps their colors honest: as pH creeps above neutral toward alkaline, the iridescence dulls even when the fish still feed eagerly.
Holding the range week to week
Set the heater around 78 degrees and give a long tank two smaller heaters or one well-placed unit near the filter outflow so warmth spreads instead of pooling at one end. Test pH after the tank cycles and monthly after that, and buffer acidity the gentle way with driftwood rather than bottled drops that wear off and bounce the number back. Weekly partial changes with temperature-matched water suit Congos well; their size means they produce more waste than small tetras, and fresh water is what keeps a pH near 6.5 from sliding lower as waste acids build.
Reading the school
A content Congo school cruises the open middle water, males displaying side by side with fins extended. Skittish darting, torn fin edges, or fish refusing food after a change usually mean the new water differed in temperature or pH from the tank, not that the fish are ill. Give them subdued light and plant cover at the back so they feel secure enough to show the behavior you are reading.
Current, cover, and the long tank
Congo tetras read a tank the way a river fish should: they want a gentle current to swim against, tall plants or wood at the back to retreat into, and a long unobstructed run in front for the school's evening cruises. Position the filter outflow along the length of the tank rather than across it, and the school will line up into the flow with fins spread, the posture that shows their water and their layout are both right. Feed a varied diet with some vegetable matter and small live or frozen foods when available, because condition and color in this species are built as much at feeding time as at the heater, and a well-fed male in correct water is unmistakable.
Sharing the established heater setting and test results in KinsLe Family Sharing means a family member doing a holiday water change can match the established numbers instead of improvising them.