A little water has no memory
Temperature stability is mostly about volume. A large aquarium holds so much stored heat that a cool night barely registers; a small fry tank may hold only a few litres, and that water gives up its warmth to a cooling room within hours. The result is a daily saw-tooth: comfortable at bedtime, chilly by dawn, warming again through the day. Fry live at the sharp end of that swing because their small bodies cannot buffer it, and repeated chilling slows growth and feeding long before it kills outright. So the fix is not a bigger blast of heat — it is slowing the loss and levelling the cycle.
Placement and insulation do the heavy lifting
Start with where the tank sits. Move it away from windows, exterior walls and doors that open to the night air, and out of any draught line. Fit a lid or cover glass: evaporation is a powerful cooler, and an open fry tank loses heat from its surface all night. Foam board behind the back glass, under the tank, and — if the room is genuinely cold — along the sides turns the setup into a small insulated box, and it costs almost nothing. A trick keepers use for very small nursery tanks is to stand or float the fry container inside a larger heated aquarium, letting the big volume of warm water act as a jacket that simply cannot cool quickly.
The heater and the thermometer have to agree
Run a small thermostatic heater sized for the tank rather than a powerful one borrowed from a bigger setup: an oversized heater overshoots in small volumes, trading a night-time dip for a noon spike. Check that it actually cycles on as the room cools, and place it where the filter's flow carries warmed water around the tank. Read the temperature with an in-water thermometer, not a stick-on strip, which mostly reports the glass and the room. Check morning and evening for a few days after any change, and judge success by the size of the swing, not by hitting one magic figure: a steady temperature a shade off target is far kinder to fry than a perfect average made of daily peaks and troughs.
When it still drifts
If the morning reading still sags, work through the unglamorous causes: a heater quietly failing, a lid left ajar after the evening feed, insulation missing on the side facing an outside wall. Change one thing at a time and judge it over two nights, not one — adjusting the heater, moving the tank and adding insulation all at once leaves you not knowing which change worked, and overshooting into a warm swing is no kinder to fry than the cold one was. Keep a spare small heater if fry are a regular project, because heaters tend to fail during the coldest week, not the warmest. Throughout, the routine only works if everyone in the house follows the same one. Family Sharing through KinsLe means a chosen relative can be given access to its profile ahead of a dawn check.