Give them the reason, because the rule alone will not survive contact
At some point in a fortnight's sitting, the tank will look less than perfect — a green tinge, a film of algae, water a touch cloudy after feeding. A sitter armed only with 'never change the water' will stand in front of that glass, feel the pull of being helpful, and start wondering whether this is the exception. A sitter who knows why will not. So take two minutes before you leave and explain it in plain terms: the fish are not living in water that happens to be in the tank; they are living in that water — its temperature, its chemistry, its balance — and their bodies are tuned to it. Replacing half of it in one go with water from the tap is like changing the air in a room to a different pressure: the shock lands on the fish within hours, not days.
Explain what the filter actually is
Most non-fish-keepers picture the filter as a strainer that catches dirt, which makes changing the water sound harmless — the strainer will catch up. Reframe it: the filter is a colony of living bacteria that eat the fish's waste, and it lives as much in the water conditions as in the sponge. A sudden large change can stall that colony with temperature and chemistry swings, and a stalled colony means the waste from every subsequent feeding has nowhere to go. Cloudy water, in this frame, stops looking like dirt to be removed and starts looking like what it usually is during a sitting fortnight: a mildly fed, mildly green, fundamentally stable tank having an ordinary week. That reframe is the single most protective sentence in the whole briefing.
Replace the forbidden task with two permitted ones
People follow 'do this instead' far better than 'never do that', so give the urge to help somewhere safe to go. Task one: top-ups. Leave a labelled container of treated water, prepared by you, beside the tank, and authorise the sitter to replace evaporation losses up to the tape mark on the glass with that container and nothing else — small, matched, boring additions that change nothing the fish can feel. Task two: the phone call. Anything that worries them — a fish on the bottom, a strange smell, water that has gone from clear to soup — earns a photo and a message to you the same day, and you, who know the tank, decide whether anything happens next. Their job is excellent witnessing; the deciding stays yours.
Name the one true exception, and script it
There is exactly one scenario where water and a sitter's initiative meet: a major leak, a cracked seam, or something toxic — a spray, a dropped chemical, a child's experiment — going into the tank. Even then, the script is not 'do a water change'; it is 'call me, and if you cannot reach me, call the emergency contact on the sheet, and do what they say'. If a fish must be moved to survive the hour, the instruction is the prepared bucket and tank water dipped from the aquarium itself, never fresh tap water. Writing this exception down matters because a sitter who has been told 'never, no exceptions' may freeze in the one moment initiative was genuinely needed; a sitter with a script acts, in the right order, fast.
Put the rule where the temptation happens
Verbal briefings fade by the second visit; the instruction has to live at the tank, in the routine the sitter actually opens. Put the water rule at the very top of the sitter's notes, in the plainest wording you have: 'Do not change the water. Top up only from the labelled container. If anything looks wrong, photograph it and message me.' Placing that rule at the head of the routine on your KinsLe Care Pass means it is the first thing read at every visit, attached to the tank's own details, rather than a line buried in a chat thread from before you left.