What leaves, and what stays behind
Evaporation is selective: only pure water escapes from the surface, while every mineral dissolved in it stays in the tank. That single fact explains both halves of good top-up practice. Leave a tank untopped for a week and the waterline falls, yet the remaining water is quietly more concentrated than when you started — hardness and salinity creep upward even though nothing was added. Then a single large top-up swings everything back at once: temperature dips, chemistry lurches, and sensitive fish and shrimp feel the change as a shock. Small, frequent additions keep the tank hovering at its normal concentration and temperature, so the fish experience top-ups as nothing at all, which is precisely the goal.
Choosing the water you add
The safest default for a freshwater tank is tap water treated with a dechlorinator, warmed to roughly tank temperature by mixing or by standing in the room until the chill is off. Because minerals never evaporate, keepers with very hard tap water sometimes top up with reverse-osmosis or distilled water instead, replacing only the pure water that left and holding hardness steady across the weeks — ordinary water changes then remain the moment to refresh minerals. Marine tanks follow the same logic with one rule that surprises newcomers: top up with fresh water, never mixed saltwater, because the salt stayed in the tank and adding more would push salinity up with every jug.
Pouring so nothing notices
Technique matters at the moment of pouring. Add the water slowly, over the filter outflow or against a rock or the glass, so it mixes in a gentle sheet instead of arriving as a cold plume that fish blunder through. A jug with a narrow spout, or a cup floated and tipped gradually, beats a tipped bucket. If the level has fallen further than usual — after a holiday, say — resist restoring it in one go: bring it up in two or three additions across the day, letting temperature and chemistry drift back rather than snap. Never top up straight from a hose or tap without dechlorinating, and keep the jug dedicated to the tank so no cleaning residue rides along.
The waterline habit, and help while you are away
Mark the normal level discreetly — a small line on the back glass or a mental note against the trim — and the daily glance at the tank includes the level without any measuring. Evaporation speeds up with open tops, warm rooms, surface agitation and winter heating, so the right rhythm is whatever keeps your mark covered: for many tanks that is a small jug every day or two rather than a bucket weekly. Travel is when top-ups get skipped, and it is exactly the job a sitter can do perfectly with clear instructions — how much, which jug, where the mark sits. Putting that routine in a KinsLe Care Pass hands your sitter the level, the jug and the dechlorinator steps in one place, so the tank they return to you is the tank you left.