Fry hunt by eye, and that sets the lighting brief
Most newly hatched fry are visual hunters from their first free-swimming day. They spot food by movement and contrast — a baby brine shrimp twitching, infusoria drifting through a shaft of light — and in dim water they simply find less of it, which is one reason broods raised in gloomy tanks grow unevenly. But the answer is not brightness. Fry have no eyelids and no shade-seeking sophistication yet; a powerful light blasted over a bare tank leaves them stressed and hugging the bottom or the glass. The target is the middle: light soft enough to live under all day, strong enough to hunt by.
Gentle and diffused beats bright and direct
A modest aquarium light at low setting, a desk lamp aimed at the wall rather than the tank, or simply good room light all work for a fry tank. Floating plants are a natural dimmer — a partial cover of water lettuce or salvinia breaks the light into dappled patches, so fry can feed in the lit water and retreat into shade, exactly the choice they would get in nature. If your light has a ramp or sunrise setting, use it; if not, switching the room light on for a few minutes before the tank light softens the transition.
Never snap the lights on in a dark room
The most stressful lighting event in a fry tank is the instant one: full light at midnight in a black room sends fry scattering into the glass, and repeated shocks cost energy they should be spending on growth. Build a soft start and a soft finish into the day — room light first, tank light a little later, and the reverse at night. This matters most in the first week, when the fry are at their most fragile and their feeding is at its most frequent. A timer on the tank light makes the rhythm identical every day, which the fry settle into noticeably.
Use light to gather the food where the fry are
Live food moves toward light, and you can exploit that. Baby brine shrimp and infusoria both drift toward a lit area, so position the light over one end of the tank and feed there: the food concentrates in the bright patch and the fry learn that this corner is the restaurant. Drop food into the lit zone rather than scattering it through shaded plant cover where it will die uneaten and foul the water. Watching a feed in good light also tells you the truth about appetites — orange bellies on brine-shrimp-fed fry, active hunting, no food left sinking after a few minutes.
A long, calm day while they are tiny
Because fry need three to five feeds a day, a longer light period in the first weeks simply gives them more hunting hours and spreads those feeds comfortably across the day. Many keepers run the fry tank light noticeably longer than the display tank's while fry are tiny, then ease back toward a normal day length as the young fish grow and move onto fewer, bigger meals. Keep the dark period properly dark and consistent, though — fry rest in it, and irregular nights undo the calm the routine builds. Tying the light checks to the same schedule as the day's feeds and siphons in KinsLe Care Teams, with each job ticked off as it is done, keeps the light period, the meals and the cleaning all marching to one steady rhythm.