A fry diary earns its keep because fry change too gradually for memory. You see the babies daily, so a batch can drift from thriving to stalling without any single day looking different, and the written record is what turns a vague worry into an obvious pattern — growth slowed the week feeds were missed, or losses followed the week water changes slipped. Five minutes with a notebook each week, covering the same few headings, is enough to catch those turns early.
Count and size: the two numbers that matter most
Start each entry with a rough count of living fry and the size of the biggest and smallest you can see, measured against something fixed — a mark on the glass, a ruler held outside the tank, or the known size of a plant leaf in frame. The gap between largest and smallest is as informative as the growth itself: a widening gap warns that food competition is sorting the batch into winners and runts, while a narrowing gap says your extra feeding spot or grading is working. Note any losses plainly, with numbers, rather than softening them; an honest count is what lets you judge whether a change you made actually helped.
Feeding graduations and appetite
Record which foods the fry are taking and which they have outgrown. The day a batch starts striking baby brine shrimp instead of needing infusoria, or manages coarser crushed flake without spitting it, is a genuine milestone, and marking the week tells you how this batch compares with the last one. Note appetite honestly too: two or three entries saying the shy ones still hang back at feeds is your cue to change where or how you scatter food, not just to feed more of it.
Colour, fins, and sexing signs
Write down when the first colour washes through, when fin shapes lengthen or round out, and when individual fish become recognisable — the one with the dark tail, the pale one, the bold one. For species where sex shows externally, note when the first certain males and females can be picked out, because that timing drives your separation and rehoming plans later. These appearance notes feel like the soft part of the diary, but they are often the entries you reread most, and they anchor the batch's story when juveniles move on to new tanks and new owners.
Behaviour and social changes
A weekly line on behaviour catches what size cannot. Note where the fry spend their time, whether the group still shoals or is spreading out and claiming corners, any chasing, nipped fins, or a fish that has started hiding through feeds. In species raised together only for a while, like young bettas, the week's behaviour note is frequently the entry that decides a separation — the diary shows the squabbling building over three entries before it becomes damage, which is exactly when acting is cheapest and kindest.
The tank facts behind the growth
Close each entry with the conditions that produced the week: temperature range, how many water changes actually happened, any new equipment, moved tanks, illnesses in neighbouring tanks, or treatments used anywhere in the fish room. Growth never happens in a vacuum, and when a future batch stalls you will flip back to find what the good weeks had in common. If fry ever need a vet's input, this section is also what turns your appointment from guesswork into a history — what changed, when, and what the fish did next.
KinsLe Pet Passport gives that diary a permanent home, so the batch's background travels whenever the young fish do — to a bigger tank, a fellow keeper, or a buyer who wants the full story.