Start with the feeding response
The first feed of the day is a remarkably honest health check you own. Healthy fry come looking: they rise, dart at particles, and keep hunting for a few minutes, and afterwards their bellies look gently rounded. Watch the edges of the group rather than the middle. Babies hanging at the surface, sitting on the floor, or drifting with fins clamped while their siblings feed are telling you something a day before it becomes obvious. Note, too, how much food is left after a few minutes — fry that leave food uneaten are either being offered too much or losing their appetite, and those two causes need opposite responses from you.
Growth is a comparison you make across days
No single glance tells you whether fry are growing; the skill is comparing today with your memory of earlier in the week. In a healthy batch the whole group edges up in size together, bellies fill out after meals, and bodies deepen rather than just lengthening. Expect some spread, but watch the tail end of it: runts that stay pinched and hollow-bellied while siblings thrive are being outcompeted, not merely small, and they do better graded into a quieter group. A phone photo taken against the same corner of the glass every few days makes growth — or its absence — much easier to judge honestly than memory does.
Losses: count them, remove them, read the pattern
One dead fry in a large batch is sad but ordinary; the pattern is what matters. Remove any dead baby the moment you spot it, because a decomposing fry in a small, warm, heavily fed tank fouls the water for everyone else. Keep a running sense of the count. A stable trickle is background; a sudden cluster of losses in a day is your earliest alarm, and it usually points at the water — test for ammonia, check the temperature, and do a partial water change before you go looking for diseases. Losses that follow a water change, a new food, or a hot night each carry their own clue, which is why the sequence matters as much as the number. Jot the day's count, the food offered and anything you changed — heater adjusted, new plant added, water changed — in a few words each evening. A single day's note tells you little; a fortnight of them turns a worrying cluster into a readable story with a visible cause.
The rest of the two-minute scan
While the batch feeds, widen the look. Fry gasping at the surface film suggest low oxygen or ammonia; a sponge filter that has stopped bubbling explains both. Cloudy water, a sour smell, or a film on the surface all say the feeding has crept ahead of the filtration. Glance at the thermometer and at any fry rubbing or flashing against the glass. None of this needs equipment beyond your eyes, and done daily it turns most fry disasters into small corrections made a day early. On mornings when a batch seems off in a way you cannot quite name, put a follow-up question to KinsLe’s Kinny; he answers using the KinsLe Answers library.