Aquarium Water Changes: How Much, How Often and How To

Water changes are the single most important maintenance job in a fish tank, and the most underrated. No filter, however good, removes the nitrate, phosphate, hormones and dissolved organics that build up every day. The only way to get rid of them is to take old water out and put fresh water in. This guide covers how much water to change, how often, how to do it step by step without stressing your fish, and the mistakes to avoid.

Why water changes matter

In a cycled tank, the bacteria in your filter convert the ammonia your fish produce into nitrite and then into nitrate. Nitrate is far less toxic, but it keeps accumulating: above 40-50 ppm it stresses fish, weakens their immune system and feeds algae. Old water also slowly loses minerals and carbonate hardness (KH), which can lead to sudden pH crashes, and it fills up with organic compounds that turn it yellowish.

A water change fixes all of that at once: it dilutes nitrate and phosphate, replenishes minerals and trace elements and keeps your parameters stable. If you are not sure how this process works yet, read our guide to cycling an aquarium first, because an uncycled tank needs to be handled differently.

How much water to change and how often

A 20-30% weekly change is a solid baseline for most community tanks, but the ideal amount depends on stocking, plants and what your test kit says. Use this table as a starting point:

Tank type	Recommended change	Notes
Tropical community tank, normal stocking	20-30% weekly	The standard baseline
Heavily planted, lightly stocked	15-25% weekly or every 10 days	Plants use up some of the nitrate
Goldfish or large cichlids	30-50% weekly	Very heavy waste producers
Nano tank (under 8 gallons / 30 L)	20-25% weekly, never skipped	Small volumes go bad fast
Shrimp tank	10-15% weekly	Gentle changes; shrimp hate sudden swings
Tank still cycling	Only if ammonia or nitrite spike	Frequent changes if fish are already in

The best way to fine-tune your routine is to test. If nitrate stays below 25-40 ppm between changes (below 20 ppm if you keep cherry shrimp or other sensitive species), your schedule is right. If it climbs higher, change more water or change it more often, or check whether you are overfeeding. Our guide to nitrite and nitrate goes into more detail.

What you need

• A gravel vacuum (siphon): drains water and pulls debris out of the substrate at the same time. There are manual models with a squeeze bulb and battery-powered ones for small tanks. You can compare options in our roundup of aquarium gravel vacuums.
• One or two buckets used only for the aquarium, never washed with soap or household cleaners.
• Water conditioner: neutralizes chlorine, chloramine and heavy metals in tap water. Brands such as Seachem, API, Tetra, Fluval or Sera make highly concentrated versions.
• A thermometer to check the temperature of the new water.
• A test kit, at least for nitrate, so you know your routine is working.
• An old towel, because there are always drips.

If you change a lot of water, a hose-style water changer that connects to a faucet saves carrying buckets. Just remember to dose conditioner for the full tank volume before refilling.

How to do a water change step by step

1. Unplug the heater and the filter. A heater can crack if it is exposed to air while hot, and a filter should never run dry.
2. Remove visible debris: dead leaves, leftover food and any algae you have scraped off the glass.
3. Vacuum the gravel. Push the siphon tube into the substrate, let the gravel tumble and drop, and work your way across in sections. On sand, hover just above the surface so you don't suck it up. You don't need to vacuum the whole bottom every week: rotate areas and stay clear of plant roots.
4. Stop once you have removed the planned amount. Mark the 25% level on the glass so you don't have to work it out every time.
5. Prepare the new water. Fill the bucket from the tap and mix hot and cold until it is within 2-3 °F (1-2 °C) of the tank. Check with a thermometer, not your hand.
6. Add water conditioner dosed for the bucket volume (or for the whole tank if you refill straight from a hose, as the label explains).
7. Pour slowly onto a plate, a rock or your hand so you don't blast the substrate or uproot plants.
8. Plug the filter and heater back in and make sure everything is running.

Temperature, conditioner and water source

Temperature is what fish feel most: a 5-7 °F (3-4 °C) difference during a big change causes stress and can trigger diseases such as ich. In a tropical tank at 75-80 °F (24-27 °C), the new water should be in the same range. In coldwater tanks, such as those for goldfish, it just shouldn't be much colder than the tank.

Always add conditioner, even if your water barely smells of chlorine. Many US utilities use chloramine, which does not gas off if you let the water sit. Dose for the volume of new water; a slight overdose is harmless, but never underdose.

If your tap water is very hard and you keep soft-water fish, you can blend it with reverse osmosis (RO) water. Always use the same ratio so pH and hardness don't swing from week to week. Pure distilled or RO water that hasn't been mixed or remineralized is not suitable for fish: it has no minerals and destabilizes pH.

Common water change mistakes

• Changing 80-100% of the water at once: it causes a massive parameter shock. Only justified in emergencies, and even then with carefully matched water.
• Skipping changes for weeks, then doing a huge one: fish have slowly adapted to degraded water and a sudden switch stresses them. If you have fallen behind, get back on track with several 20% changes every other day.
• Rinsing filter media under the tap: it kills the bacteria. Swish it in the tank water you just removed instead.
• Cleaning the filter and deep-vacuuming on the same day: you remove too much bacteria at once. Space them a week apart.
• Using soap or detergent on buckets, siphons or glass.
• Forgetting the conditioner, or adding it after the untreated water is already in.
• Only topping off evaporated water: evaporation removes pure water and leaves the dissolved solids behind, so topping off is not a water change.

Special cases

After medicating

When a treatment is finished, do a 30-50% change and put activated carbon back in the filter if you removed it, to clear out any remaining medication.

If fish are gasping at the surface

Test for ammonia and nitrite. If either is above 0, do a 30-50% change that same day with well-matched water and repeat the next day if needed.

Tanks with bottom dwellers

Corydoras and other fish that forage in the substrate appreciate a clean bottom: vacuum more carefully wherever food tends to collect.

A weekly 15-20 minute routine gives you stable water, healthier fish and far less algae. To understand what you are correcting with every change, read our guide to aquarium water parameters.

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