The water they evolved in
Chili rasboras are Borneo fish, schooling in tea-brown peat swamps and slow blackwater streams where fallen leaves stain the water and minerals are scarce. That origin fixes the shape of the answer: warm water, soft water, and acidity that would alarm a keeper of hard-water fish. In a home aquarium they do best at about 73 to 79 degrees Fahrenheit, within a natural range that runs a few degrees either side, and at a pH between roughly 5.0 and 6.5, inside a wild span that can dip lower still. They are adaptable within reason, but their red colour, tight schooling and willingness to feed all sharpen when the water resembles home.
Temperature: warm and, above all, steady
For a fish barely two centimetres long, the danger is rarely the average temperature - it is the swing. A small tank cools fast when a room chills overnight and warms just as fast near a sunny window, so sit a chili rasbora tank away from radiators and direct sun, and run a heater sized for the tank with a separate thermometer you actually read. A degree or two of drift across a day is normal; repeated drops into the sixties are not, and they show up as pale, listless fish hiding in the plants. Check the thermometer at morning and evening feeds until the pattern is familiar.
Softness and acidity without the chemicals
What the rasboras need alongside that warmth is low-mineral, gently acidic water, and the safest route there is source water, not bottled adjustments. Keepers who need softer water typically blend reverse-osmosis water with a little tap water or use a dedicated remineraliser, arriving at softness gradually. Botanicals help in the right direction too: leaf litter and alder cones release the tannins these fish live among, nudging acidity while giving the school dark cover to feel brave under. What to avoid is pouring in pH-lowering chemicals to hit a number - the reading moves, then rebounds, and the fish ride every swing.
Test small, change slowly
A liquid test kit earns its place with this species because the differences that matter - a pH of 6.8 creeping toward 7.4 as a filter matures, softness rising with every topped-up litre of hard tap water - are invisible to the eye. Test weekly, log the numbers, and make any correction in small steps spread over days: a slightly larger share of soft water at the next change, never a full reset. Top up evaporated water with soft water too, since topping up with tap water steadily concentrates minerals in a small tank.
When the numbers are simply home
Once the tank has run at its temperature and pH for a few settled weeks, those readings become part of the fish's identity - the water any quarantine tank, temporary tub or future upgrade should be matched to before the school moves. That is exactly the sort of detail that gets lost on a sticky note. Writing the rasboras' settled temperature, pH and softness into their Pet Passport on KinsLe keeps the figures with the fish themselves, so whoever sets up their next tank - you in a hurry, or a sitter while you are away - starts from the water they already thrive in.