A log scaled to a very small fish
The least killifish is one of the smallest livebearers in the hobby — females stay around the length of a thumbnail — so the weight half of a health log simply does not apply. No home scale resolves a fish this size, and scooping one out for the attempt can injure it. The health half, however, is well worth keeping, precisely because this species is easy to overlook. In a planted tank these fish drift quietly among the stems, and a female slowly losing condition can vanish from your attention for weeks. A short written entry each week makes you stop, find every fish, and actually look.
How this species' breeding shapes the log
Least killifish do not drop one large brood the way many livebearers do. A female carries developing young at different stages and releases a few fry at a time across many days, so a healthy tank shows a near-continuous trickle of tiny newborns rather than a monthly event. Your log should capture that rhythm: how many fry you spotted this week, whether the females look comfortably rounded, and whether any female has stayed slim and produced nothing for an extended stretch. That pattern, recorded over a couple of months, is the clearest evidence you can have that the colony is thriving — or the earliest hint that conditions, food, or an ageing female need attention.
What a useful entry contains
Keep it brief enough that you will actually sustain it. Count the adults you can find and note where they are spending time, since a fish that abandons the plant thickets for a corner is behaving out of character. Record feeding response carefully: least killifish need appropriately tiny foods, and a fish repeatedly missing out because tankmates reach the food first will thin while the group looks fine. Note body shape from above — smooth and filled behind the head is what you want — plus any visible fry, any dead fish found, and routine facts like water changes and the foods offered that week. In a nano tank, also note evaporation top-ups, because small volumes drift faster than large ones.
Photos beat guesswork at this size
Judging the condition of a fish this small from memory is unreliable, but a macro phone photo is surprisingly revealing. Photograph the females against the same plant or piece of wood each week if they cooperate, or take a short burst at feeding when they rise for food. Compared over time, those images show whether a female's outline is holding or hollowing, and whether juveniles are growing on pace with their siblings. Attach the best frame to each log entry so the written words and the visual evidence stay paired.
When to escalate beyond the notebook
Seek an aquatic veterinarian's input when a least killifish stops feeding, when a female who has been producing fry goes slim and stays that way while also losing her appetite, when you find deaths on consecutive weeks, or when several fish show clamped fins or laboured movement together — that group pattern usually points at the water, so test it the same day and record the readings. A single shy day after a big rearrangement of the tank is normal for this species; a trend across three entries is not, and the log is what proves the trend.
Giving tiny fish a proper record
It is tempting to treat fish this small as decoration, but their care history deserves the same order as any other pet's. Entering each week's notes into KinsLe's Pet Passport keeps your least killifish's observations, photos, and water readings together in one packet, ready for a vet conversation or a careful handover to a new keeper.