When fish look ill, the urge is to reach for a treatment. Resist it until the husbandry has been reviewed, in order, because most aquarium illness traces back to a husbandry factor: something in the water, the filtration, the stocking, or the feeding that weakened the fish before any disease took hold. Treating before checking medicates blind — the ailment may lift briefly while the cause remains in the tank, ready to produce the next case. The review below is the deliverable: a fixed sequence, worked through honestly, with each result recorded rather than assumed.
One: Test the Water
Test first, and test in full: ammonia, nitrite, nitrate, pH, and temperature, each recorded as an actual number. Ammonia and nitrite expose a filter that is not coping or a cycle that has broken down; nitrate shows whether water changes are keeping pace with the waste the stock produces; pH reveals drift or a sudden swing from the water the fish are used to; temperature confirms the heater is holding the set point. Compare the readings with your logged history, not with memory, because a parameter that has slid gradually over a month looks innocent in a single test and obvious in a series. Until this step is done and written down, nothing else in the review can be interpreted, since sick-looking behaviour in bad water is a water problem until proven otherwise.
Two: Check the Filtration
Move from the water to the machine that maintains it. Check the flow first: is the filter turning the water over at its usual rate, or has flow slowed to a drift? Then examine the media state — media clogged with sludge restricts flow and harbours waste, media recently replaced wholesale may have taken the beneficial bacteria with it, and media rinsed under untreated tap water can lose them just as fast. Confirm the filter runs continuously, with no timer gap or overnight switch-off in the routine, and check intakes for blockages from plant leaves or debris. Many mystery illnesses resolve at this step, in the discovery that the filter has been half-choked for weeks and the water test was simply the first place it showed.
Three: Count the Stocking and the Feeding
Stocking and feeding are one question asked twice, because both set the waste load the system must carry. Count the fish actually in the tank, including fry that have grown since the tank felt lightly stocked, and weigh that against the tank's capacity and the filtration you have just checked. Then review feeding amount honestly: how much goes in, how often, and how much uneaten food is left to rot in the gravel or foul the surface. Overfeeding is the most common finding in this whole review, and it hides behind generosity — a pinch extra at every meal, food given by several household members, or rich foods offered daily that the fish cannot finish. Note any new food introduced recently as well, since a feeding change belongs in the same picture as a stocking change.
Four: Temperature Stability and Recent Changes
Last, review stability and history together. Check temperature stability beyond the single reading: has the tank swung between day and night, sat near a radiator or a sunny window, or endured a heater fault in the past fortnight? Swings stress fish even when the average looks right. Then list recent changes and new additions in order: new fish, new plants, new ornaments, a changed water source, a bigger water change than usual, a different food, or work done near the tank. New additions in particular can import trouble or upset an established hierarchy, and a symptom that began days after an addition tells a different story from one that began in an untouched tank. With all four steps recorded, the position is honest: correct what the review found, observe closely, and if signs persist in verified good conditions, that record becomes the foundation for proper veterinary or specialist advice.
KinsLe's Pet Passport keeps water tests, filter maintenance, stocking changes, and feeding notes in one dated health history for the tank, so the next illness review starts from evidence.