A guppy found dead with no marks, then another a few days later, is one of the most frustrating patterns in fishkeeping, precisely because the fish carry no visible evidence. What replaces the missing marks is a record: water readings taken now, behaviour watched closely, and each loss logged with the details that let a pattern emerge. Either possibility — the water itself or an illness moving through the tank — is one a vet or experienced aquarist may investigate, and the observations below are what help them distinguish between the two instead of guessing.
Test the Water First, and Record Numbers
Before anything else, test the water and write down actual numbers: ammonia, nitrite, nitrate, pH, and temperature, each as a reading rather than a judgement that levels look fine. A useful entry reads like this: ammonia 0.25, nitrite 0, nitrate 30, pH 7.6, temperature 26 degrees, tested in the morning before feeding. Numbers recorded this way can be compared across days and handed to anyone advising you, while a memory of the water seeming clear cannot. Repeat the tests over the following days and log each set with its date and time, because a single test is a snapshot and a creeping problem shows in the series. Note alongside the readings when the last water change happened and how much was changed, since deaths that cluster just after changes tell their own story.
Behaviour Before Each Death
Watch the survivors closely, because behaviour in the hours before a death is evidence the dead fish cannot give. Note any gasping at the surface, which points the investigation toward the water or the gills. Record flashing, where a fish flicks sideways against gravel, plants, or glass, and shimmying, where a fish hovers and wiggles in place instead of swimming freely. Hiding, clamped fins, loss of appetite, and separation from the group all belong in the note, with which fish showed each sign and for how long. If a fish dies, record what that individual was doing in the day or two before, as exactly as you can remember it. Behaviour logged fish by fish turns a vague worry into a sequence an adviser can read.
Which Fish Died, and What Changed
Keep a loss record with the same fields each time: which fish died, its age if known, its sex, and whether it was recently added to the tank. A pattern where only new arrivals die reads differently from one where the oldest fish go first, and losses among fry, adults, males, or females each suggest different lines of enquiry. Alongside the losses, record feeding — what is fed, how much, and whether uneaten food is left — and every recent change you can identify: new fish, new plants, a different water source, a moved tank, a new filter or ornament, or a change in the water-change routine. Nothing is too small to log; the change you almost omit is frequently the one an experienced eye picks out first.
Putting the Record to Work
With readings, behaviour notes, and the loss record together, the possibilities can be weighed properly: a water problem usually leaves fingerprints in the numbers or in surface gasping across many fish, while disease more often picks off individuals with behaviour changes first. That weighing belongs to a vet or an experienced aquarist, and your record is what makes their judgement possible from a distance or across a counter. Keep logging while you wait for advice, and note any day with no loss as carefully as a day with one, because the shape of the gaps is part of the pattern too.
KinsLe's Pet Passport stores the tank's test readings, stock changes, and loss record in dated order, holding a health history for the aquarium that survives long after memory of one bad week fades.