Shallow water while everything is still forming
Newly free-swimming betta fry are weak swimmers with small energy reserves, and everything they need — food at the surface film, air, warmth — sits at the top of the water. A depth of only a few inches makes each surface trip short and cheap, and it concentrates their live food in a layer the fry can actually patrol rather than diluting it through a tall column they cannot cover. Deep water at this stage does not drown fry outright so much as tax them: more energy spent travelling means slower growth, and the weakest fry fall behind first. A few inches in a modest tank also keeps the whole batch visible, so dead spots in feeding and the first signs of trouble are easy to spot at a glance.
The labyrinth organ changes what the air must be
Bettas breathe surface air through the labyrinth organ, and that organ develops across roughly the first month of life, with fry beginning to gulp at the surface as it forms. During this window the layer of air above the water matters almost as much as the water itself: it should be warm and humid, close to the water's own temperature. A fitted lid or cover glass achieves this, trapping moist air over the surface. Gulps of cold, dry room air are a recognised setback for developing fry, which is why a betta fry tank in a draughty room or under air conditioning struggles even when the water parameters are perfect. Open the lid briefly and gently for feeding rather than leaving it ajar all day.
Deepen the water gradually, in step with strength
Once the fry are feeding confidently, ranging the full tank and returning to the surface without effort, begin raising the level an inch or so at a time over several days. Each small rise asks a little more of their swimming and their developing organs without ever presenting a sudden challenge, and it gives the batch more volume, and steadier water quality comes free with that. Watch the smallest fry at every step rather than the leaders: the batch is ready to deepen when the runts cope easily, not when the biggest fry are bored. By the time the juveniles are clearly sexable and eating adult-sized foods, they can be worked up towards normal tank depth on the same gentle schedule.
Judge by behaviour, not by the calendar
Batches develop at different speeds depending on feeding, temperature and genetics, so let the fish arbitrate. Signs a batch wants more water: strong, purposeful surface feeding, exploration of the bottom with easy returns, and fry spread through the whole depth they have rather than clustered at the top. Signs to hold steady: fry resting exhausted on leaves after surface trips, or the smallest third permanently hugging the surface film. Before the level changes for a slow batch, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.