Guide
How often to change aquarium water
A typical water change is 20–30% once a week. The best way to set the amount is a nitrate test: for most fish keep it below 25 mg/l (ppm).
How much and how often
| Type of tank | Water change |
|---|---|
| Community tank | 20–30% weekly |
| Planted tank with CO2 and fertiliser | 30–50% weekly |
| Heavily stocked, large fish | 30–50% once or twice a week |
| Shrimp tank | 10–20% weekly, as shrimp dislike sudden changes |
Matching water changes to nitrate
With regular water changes nitrate settles at about weekly NO3 increase ÷ fraction of water changed.
Example: nitrate rises by 10 mg/l a week. With 30% changes it settles at about 33 mg/l, with 50% at about 20 mg/l. Measure the increase with a test right after a water change and a week later.
Preparing the water
- Tap water needs a conditioner that removes chlorine and chloramine. Letting it stand does not remove chloramine.
- New water should be within 1–2 °C of the tank.
- Hardness (KH, GH) and pH of the new water should be close to the tank's.
What to avoid
- Rare, very large water changes after a long break. In a neglected tank change the water gradually over several days.
- Cleaning the filter and the whole substrate on the same day: you lose too many bacteria at once.
- Only topping up evaporated water: only water evaporates, salts and nitrate stay.
How Aquarium Monitor helps
In water-change mode the level sensor measures how many litres you drained and refilled. Afterwards the app compares temperature and pH before and after to catch badly matched water, and water changes show up on the charts and in the history. Recurring tasks remind you of the next one.
Frequently asked questions
Should I turn off the filter and heater during a water change?
Turn the heater off if the water drops below its minimum immersion line. Turn the filter off when its intake is above the water.
Can I add water straight from the tap?
Yes, if you add a conditioner and match the temperature.
Can I change 50% of the water at once?
Yes, if you do it regularly and the new water has similar parameters. After a long break, split it into several smaller changes.
Keep an eye on your tank from your phone
The app is free and ad-free. The monitor runs on your Raspberry Pi.