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Water Change Calculator

Calculate how often and how much water to change in your aquarium based on volume, fish count and plants

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๐Ÿ‡ฉ๐Ÿ‡ฐ All calculations are based on Danish rules, rates, and currency (DKK). This calculator is translated for convenience but applies exclusively to Denmark.

Calculate water changes for your aquarium

Regular water changes are essential for a healthy aquarium. Fresh water removes accumulated nitrate, organic waste and replenishes minerals. This calculator helps you find the right water change frequency based on your aquarium's specific conditions.

Why are water changes necessary?

Even with a good filter, nitrate accumulates in aquarium water over time. Nitrate is the end product of biological filtration and can only be effectively removed through water changes. High nitrate levels (above 40 mg/l) lead to algae growth, fish stress and weakened immune systems.

Standard recommendation

The baseline is a weekly water change of about 25% of aquarium volume. This is adjusted upward with many fish relative to volume, and downward if the aquarium is heavily planted, as plants absorb nitrate.

Overstocking requires more

If fish numbers exceed the rule of thumb (1 fish per 10 litres), water change needs increase. For each fish above this threshold, the recommended percentage increases by 2 points. However, you should always consider reducing fish numbers rather than increasing water changes.

Positive effect of plants

Live plants absorb nitrate and other nutrients from the water and act as a natural filter. A densely planted aquarium can reduce water change needs by about 5 percentage points. Fast-growing plants like hornwort and duckweed are particularly effective.

Frequently Asked Questions

How often should I change water in my aquarium?

The standard recommendation is a weekly water change. For most aquariums this is sufficient to keep nitrate levels low. Lightly stocked aquariums with many plants may manage with water changes every 2 weeks, while overstocked aquariums may require 2 water changes per week.

Can I change too much water at once?

Yes, very large water changes (over 50%) can stress fish as they change water chemistry too quickly. Temperature differences, pH changes and mineral content can vary significantly between old and new water. Stick to 20-30% per change for best results. In case of acute pollution, larger changes may however be necessary.

Should the new water be the same temperature?

Yes, new water should be approximately the same temperature as aquarium water โ€“ preferably within 1-2ยฐC difference. Large temperature swings can shock fish and trigger disease, especially ichthyophthirius (white spot). Use a thermometer to check temperature and mix hot and cold water before adding.

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