The short answer for a fish this small
Keep the log, but change what goes in it. An Endler's livebearer weighs so little that a scale tells you almost nothing, and chasing a two-centimetre fish around the tank with a net to weigh it costs more in stress than the number could ever be worth. What actually protects a colony of Endlers is a written record of condition: how rounded each female looks, how brightly each male is coloured, how the group behaves at feeding, and how the fry count is trending. Written down weekly, those observations become a baseline you can compare against the moment something looks off.
Why Endlers change too fast for memory alone
Endlers breed almost continuously when males and females share a tank, so the population you are watching is never standing still. A female who looks slim this week may be heavily gravid the next, then slim again after dropping fry, and none of that is illness. Without dated notes, normal breeding cycles and genuine problems blur together — a female who stays swollen long after her tankmates have slimmed down, or a male whose vivid orange and black markings fade over a fortnight, can slip past an owner relying on memory. The log separates the rhythm of a healthy colony from the one fish drifting away from it.
What to record at each weekly check
Pick a regular moment, ideally just before a water change, and work through the same short list. Count the adults and note the fry you can see, even roughly. Look at females from above and the side: a smooth, rounded belly fits a gravid livebearer, while a pinched look behind the head or a hollow belly on a fish that is eating suggests she is losing condition. Check males for colour, because a displaying male in good health shows his pattern strongly, and persistent dullness is worth flagging. Note appetite across the group, any fish hanging apart at the surface or hiding, fin condition, and anything new in the tank — a plant, a tankmate, a filter change — so later entries can be read against what changed.
Photographs do the job a scale cannot
A phone photo taken from the same side of the glass, at the same distance, once a week gives you the closest thing to a weight curve this species allows. Side-by-side, a run of photos shows a female gradually thinning, a male's colours washing out, or a young fish failing to grow at the pace of its siblings, all far more clearly than a single glance at a busy tank. Save the clearest shot with each log entry and discard the blurry extras, so the sequence stays readable rather than becoming a camera roll you never open.
When a log entry should become a vet conversation
Most Endler entries will be routine, and that is the point — the routine pages make the abnormal one stand out. Contact an aquatic veterinarian when a fish stops eating for more than a couple of days, when several fish change behaviour at once, when you see clamped fins paired with rapid breathing, white spots, or a female who remains distended while losing weight elsewhere on her body. Bring the log itself to that conversation, because the timeline of appetite, fry drops, and tank changes is often what helps a vet narrow down whether the cause sits in the water, the food, or the fish. Test the water first whenever multiple fish look wrong together, and write those results into the same entry.
Keeping the colony record in one place
A notebook by the tank works until it gets splashed, misplaced, or left behind when the fish are rehomed or a sitter takes over. Storing the entries, photos, and water notes in KinsLe's Pet Passport keeps your Endler's whole record in a single portable packet, so the colony's history travels cleanly to a vet visit, a holiday handover, or a new owner's tank.