Doing Regular Water Changes
The core maintenance routine — how much water to change, how often, and how to do it without shocking your fish.
Also known as: Partial water change, Gravel vacuuming
Regular partial water changes are the backbone of aquarium maintenance. They remove accumulated nitrate and waste, replenish minerals, and keep water stable. Done consistently and gently, they are the single most reliable way to keep fish healthy.
What it is
Even a fully cycled tank steadily accumulates nitrate and other dissolved wastes that the biological filter does not remove. The cure is dilution: replacing part of the water on a schedule. Water changes also top up trace minerals that fish and plants consume, and remove debris from the substrate.
How much and how often. A common, reliable routine is a 20–30% water change weekly for a typical stocked community tank. Heavily stocked tanks may need more; lightly stocked, planted tanks may need less. The goal is to keep nitrate low (many keepers aim to keep it well under 40 ppm) and stable — let a test kit, not a calendar alone, guide the amount.
How to do it safely. Use a gravel vacuum (siphon) to draw water out while lifting waste from the substrate. Refill with fresh water that is dechlorinated — tap water usually contains chlorine or chloramine that harms fish and filter bacteria — and temperature-matched to within a degree or two of the tank, so the change doesn't chill or shock the fish. Adding a small amount of fresh water quickly is fine; large swings in temperature or chemistry are what cause stress.
Avoid over-cleaning. Never replace all the water at once, and don't scrub or rinse the filter media in tap water — the chlorine in it strips out the beneficial bacteria and can trigger a mini-cycle. Rinse media gently in a bucket of old tank water only when flow visibly drops. Consistent small changes beat occasional large ones every time, because a big, sudden change can swing temperature and chemistry enough to stress fish, whereas frequent modest ones keep conditions steady. If a tank has been neglected and nitrate is very high, correct it gradually over several days with a series of smaller changes rather than one huge one, so the fish are not shocked by an abrupt shift in water they had adapted to.
Worked example
A keeper with a moderately stocked 100 L tank tests nitrate at 30 ppm and rising. Each week they siphon out ~25 L (25%) with a gravel vacuum, cleaning a different area of substrate each time. They refill with dechlorinated water matched to 25 °C. Nitrate settles into the 10–20 ppm range and stays there, fish stay active and coloured, and the filter is left alone except for an occasional gentle rinse in old tank water.
Related entries
Sources & further reading
- How to Do an Aquarium Water Change — The Spruce Pets (article)
- Why and How to Change Aquarium Water — Aqueon (article)