The fish themselves add very little
Harlequin rasboras are small, deep-bodied schooling fish, rarely much over an inch and a half, and a proper school of eight or more still adds up to very little fish. Their appetites match their size: a pinch of micro-pellets or finely crushed flake, gone in a minute, is a full feeding, and a single small container of food can serve a school for a remarkably long time. Their waste output is equally slight, so filters run easily and water stays clean between changes with minimal effort. If the fish were the whole story, a harlequin school would be close to free after purchase day.
The planted tank around the school is the real expense
Harlequins are almost always kept the way they look best: in a warm, planted aquarium with subdued light and dark substrate that sets off their copper and black markings. That setting, not the fish, writes the monthly bill. A heater holds the water in the mid-seventies to low eighties Fahrenheit around the clock. A light runs every day, and if the plants are real rather than decorative, it runs long enough and brightly enough to grow them, which is a major electricity draw in a nano setup. Liquid plant food, root tablets and the occasional replacement plant are small but genuine recurring lines that rasbora-free budgets never meet.
Water and filter costs stay feather-light
Weekly partial changes for a rasbora tank are small, so conditioner use per month is modest, and the filter, whether a gentle internal unit or a small hang-on-back, sips electricity and asks only for rinsed foam rather than replacement cartridges. This is where harlequins undercut fussier fish: their light stocking means the mechanical side of the hobby stays cheap and forgiving. Testing supplies are shared, occasional and inexpensive. Taken together, the tank's services cost a small, steady amount each month, closer to running a lamp and a kettle's worth of treated water than to feeding a pet in the usual sense.
Where rasbora owners actually overspend
The leaks are aesthetic rather than necessary. Chasing ever-brighter lighting to make the copper color pop, dosing multiple plant supplements where one would do, and replacing plants that a simpler species choice would have kept alive all inflate a budget the fish never asked for. Another is school arithmetic: because individual rasboras are inexpensive and charming, tanks quietly accumulate other nano species, and each addition shares the same water but multiplies food variety and care time. None of these spends is large; they are simply the difference between the modest baseline and what an enthusiastic planted-tank hobby grows into.
A fair monthly expectation
Budget for a small planted tropical tank, not for the fish: heater and light electricity first, conditioner and plant care second, food a distant third, and you will land close to the truth every month. The school itself will repay that modest sum with constant, calm movement and some of the prettiest color in the freshwater hobby. For school size against tank size while you are still at the planning stage, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.