Start with a batch code, not good intentions
The moment a spawning happens, give it a code before you do anything else: the parents' names plus a letter for the batch, so the second spawning from the same pair becomes batch B. Write that code on a strip of tape on the tank glass the same day. Memory feels reliable on the evening the eggs hatch and is worthless a fortnight later, when two lots of fry of the same species hang in the water looking exactly alike. The code is the whole system — everything else in the log hangs off it.
What one log page per batch needs
Give each batch its own page or notebook section headed by the code. On it, write the parents first — both of them, not just the showy male — because parentage is the one fact you can never reconstruct once the fry are mixed. Add the hatch day, roughly how many fry you started with, the first foods you offered and when you moved to the next food, every tank move, and any losses or deformities you culled. Short factual lines beat essays; you are writing for a tired future you who wants the answer in five seconds.
Match the label on the tank to the page
Fry change tanks as they grow, and every move is where lineages get lost. When a batch moves to a grow-out tank, the tape label moves with it, and the log page gets one new line: which tank it went to, and whether the batch was split. If you split a batch, the two halves become batch A1 and A2 on both the glass and the page, so a fish sold, swapped or kept for breeding can always be traced back to its parents in one look.
Use the log to make better pairings
A breeding log earns its keep at the next spawning decision. The page tells you which pairing gave strong, even fry and which threw runts or bent spines, which first food a batch took eagerly, and which pair produced fry that coloured up early. Without it, breeders end up repeating a poor cross because the fry looked fine at four weeks, or accidentally line-breeding fish that share parents. Two minutes of notes per batch turns guesswork about your own stock into actual knowledge of your lines.
Keep the parents' records with the fry notes
The fry pages only make sense while they sit with the adults' information — ages, where the parents came from, and their own batch codes if you bred them yourself. Kept together, the whole history of a line can be handed to a buyer or a fellow breeder in one piece instead of reconstructed from memory at the door. Holding the parents' details and each batch's parentage notes in KinsLe's Pet Passport keeps that family history in one packet, ready whenever a fish changes hands.