Evaporation never stops, so topping up works best as a plan rather than a reaction. When you top up only after noticing a low water line, the level swings up and down, equipment splashes, and water chemistry shifts with it. This planner turns your daily evaporation and your container size into a simple routine: how many top-ups per week, how much each time, and how many days between them.
Start with your daily evaporation figure. If you used the evaporation calculator, enter that number here. If not, measure it yourself: mark the water level, wait a week without topping up, measure how far it fell, and divide the lost litres by seven. A bucket of known size makes the measurement easy. Guessing evaporation usually underestimates it, because a few millimetres of level drop across a wide tank is more water than it looks.
Container size shapes the routine more than people expect. A five-litre jug suits a small tank with low evaporation, because one jug covers a week. A large tank losing a litre a day needs a bigger container or more frequent trips. The planner rounds your weekly need up to whole container fills, so you always prepare slightly more than the minimum. That margin covers hot days, dry rooms, and weeks when you miss a day.
To use the tool, enter daily evaporation in litres and your container size in litres, then press the button. The result gives the weekly evaporation total, the number of top-ups per week, the amount per top-up, and the gap in days between top-ups. For example, a tank losing zero point seven litres per day needs about four point nine litres per week. With a five-litre container, that is one top-up per week of about four point nine litres, roughly every seven days.
Small, frequent top-ups beat large, rare ones. A tank topped up every two days holds a steadier level than a tank topped up once a week, and steady levels keep filters quiet and heaters submerged. If the planner suggests one top-up per week but your filter splashes by day five, split the amount into two smaller top-ups anyway. The planner gives a minimum routine; your equipment tells you whether to split it further.
Prepare top-up water in advance. Fill your container, treat it for chlorine and chloramine if you use tap water, and let it reach room temperature before adding it. Cold top-up water shocks fish and can crack a heater that is running. Keep the container covered, clean, and used only for aquarium water, so it never picks up soap or chemicals. Label it clearly if other people in the house might use it for cleaning.
Top-up water choice matters over months. Evaporation leaves minerals behind, so topping up with hard tap water slowly raises hardness and total dissolved solids. Regular partial water changes dilute those minerals again, which is why top-ups never replace water changes. If your tank needs soft water, top up with reverse-osmosis water instead, and remineralise only during water changes. Test hardness monthly if you top up with tap water.
Build the routine around your week. Pick fixed days, such as Sunday and Wednesday, and keep the container where you will see it. A routine that depends on remembering fails in busy weeks, while a fixed-day routine survives. If you travel, leave written top-up instructions and a pre-filled container for whoever feeds the fish. An automatic top-up unit is worth considering for large or open-top tanks, but even then, check the level yourself weekly.
Automatic top-up systems suit tanks with high evaporation, but they need care. A float valve or sensor pump adds water as the level falls, which keeps the level perfect. The risks are a stuck valve that overfills, or a sensor that fails and runs a pump dry. Use a small reservoir, not a direct mains connection, so a failure can only add a limited amount. Check sensors monthly for algae or salt build-up that can jam them.
Watch for signs that your plan needs adjusting. A filter that starts splashing mid-week, a tidemark that grows fast, or a heater that peeks above the water all mean top-ups are too infrequent or too small. Rising hardness on tests means top-up water is adding minerals faster than water changes remove them. Seasonal changes matter too: re-run the planner in winter, when dry heated air raises evaporation sharply.
Multiple tanks need separate plans, because evaporation differs by surface, temperature, and lid. A covered tank may need one top-up per week while an open-top tank beside it needs three. Do not copy one routine across tanks. Measure each tank, plan each one, and keep a small card on each tank with its top-up days and amount. That simple card prevents the most common multi-tank mistake, which is topping up the wrong tank by the wrong amount.
Finally, remember that a top-up plan is a support routine, not a substitute for maintenance. Water changes remove waste, replenish minerals, and reset chemistry in ways that top-ups cannot. Pair this planner with a weekly partial water change, a daily level glance, and a monthly hardness test. Stable water levels, steady chemistry, and quiet equipment together make fishkeeping calmer for you and healthier for your fish.