Eggs develop on a schedule that swings disrupt
A tetra egg is a developing embryo running on temperature. Within the species' comfort range, warmth sets the speed of development: steadier warmth, a hatch that arrives on time with the whole clutch emerging together. What eggs tolerate badly is change. A tank that drifts up through the afternoon sun and cools overnight forces the embryos through repeated expansion of that schedule, and the result is the classic failed tetra spawn — eggs that cloud over, hatch too early as weak larvae, or produce fry with bent spines that never swim properly. The number on the heater matters less than the flatness of the line across the whole incubation.
Small water swings fast, so protect the volume
Tetra hatching setups are usually small — a little bare tank with a sponge filter — and small volumes of water change temperature far faster than large ones. A two-degree room swing that a big display tank would shrug off overnight can pass straight into a hatching tank before morning. Site the hatching tank accordingly: an interior wall, away from windows, radiators, exterior doors and direct sun. Some breeders stand the little tank inside a larger water bath or keep it in a temperature-stable cupboard room for the two or three critical days, purely to borrow thermal mass.
Equipment: one reliable heater, one honest thermometer
Use a single heater you trust, sized gently for the small volume — an oversized heater in a tiny tank overshoots in short bursts, creating exactly the sawtooth pattern you are avoiding. Check the tank with a thermometer you have verified against another, not just the heater's dial, because heater dials are routinely a degree or two optimistic. Read the temperature at the same times each day during a spawn, morning and evening, so you see the drift pattern rather than a single snapshot. If the readings wander, fix the room or the placement before blaming the fish.
Larvae are even less forgiving than eggs
Hatching is not the finish line. Newly hatched tetra larvae hang motionless, living on their yolk sacs, and temperature shocks in these first days — including the classic accident of a water change with cooler water — can kill a whole brood that hatched perfectly. Any water you add to a larvae tank should be matched degree for degree, poured in slowly, and kept small in volume. Keep the tank dim as well, since many tetra eggs and young larvae are light-sensitive, and let the sponge filter's gentle movement do the circulation work without chilling the surface.
Build a record, because the next spawn is coming
Tetras in condition spawn repeatedly, and each attempt teaches you something about your room, your heater and your pair — but only if you wrote it down. Note the temperature morning and evening, the day the eggs were laid, when they hatched, and how the larvae did; a clutch that failed after a warm afternoon tells a different story from one that failed in a cold snap. Keeping those spawn records together in the KinsLe Pet Passport for the breeding fish means the history is in one packet when you review it, share it with another keeper, or take advice about the pair — and each spawn starts from evidence rather than a hopeful memory.