Aquarium Water Change: How Much, How Often, and How to Do It
The single most important maintenance task, done wrong by most beginners. How much, how often, and the safe way to use Indian tap water.
If you do only one thing for your aquarium, make it a regular partial water change. It removes nitrate that the filter cannot, replaces trace minerals, stabilises pH, and dilutes the dissolved organics that stress fish long before they cause visible symptoms.
How Much and How Often
| Tank situation | Change amount | Frequency |
|---|---|---|
| Standard community tank, normally stocked | 20-30% | Once a week |
| Lightly stocked, heavily planted | 15-20% | Every 10-14 days |
| Heavily stocked (cichlids, goldfish, large fish) | 30-40% | Twice a week |
| Nano tank under 20 litres | 20-25% | Twice a week (small volumes swing fast) |
| Tank still cycling with fish in it | 25-30% | Daily until ammonia and nitrite read 0 |
| Ammonia or nitrite spike | 30-50% | Immediately, then daily until stable |
| Breeding or grow-out tank with fry | 20% | Daily or alternate days, gently |
💡The right answer for your specific tank comes from a nitrate test, not a rule. Change enough water, often enough, to keep nitrate below 40 ppm at all times. If nitrate is climbing week to week, change more or change more often.
How to Do a Water Change: Step by Step
- Fill a clean bucket (one that has never held soap or detergent) with tap water.
- Add water conditioner at the dose on the bottle. Indian municipal supplies mostly use chloramine, which does not evaporate off and must be neutralised chemically.
- Check the temperature. New water should be within 1-2°C of the tank. In winter, mix in a little hot water; do not use a kettle full of boiling water directly.
- Unplug the heater if the water level will drop below it. Never leave a heater running dry.
- Siphon out the target amount, gravel vacuuming one third of the substrate as you go.
- Pour the new water in slowly, aimed at the glass or onto a plate resting on the substrate.
- Plug the heater back in, confirm the filter is flowing, and check that fish are behaving normally.
Indian Tap Water: What You Need to Know
Chlorine and chloramine
Older advice says to leave water standing overnight to let chlorine gas off. That worked when supplies used plain chlorine. Most Indian cities now dose chloramine, which is stable and will not evaporate. A dechlorinator (Seachem Prime, API Tap Water Conditioner, or any local brand that states it neutralises chloramine) is mandatory.
Borewell and hard water
Much of Chennai, Hyderabad and Bengaluru runs on hard borewell water with high TDS. Goldfish, koi, guppies, mollies, platies and African cichlids do very well in it. Discus, cardinal tetras, wild bettas and most soft water species struggle. Choose fish that match your water rather than fighting your water with chemicals, which causes unstable swings and kills more fish than hard water ever does.
RO water
Never use pure RO water on its own. It has no minerals and no buffering, so pH crashes unpredictably. If you need soft water for discus or wild caught species, mix RO with tap water (commonly 50:50 or 70:30) or use a remineralising salt. Test and keep it consistent.
Mistakes That Cause Real Damage
- 100% water changes. This strips the biological system and shocks the fish with an instant change in every parameter at once. Never do this on an established tank.
- Skipping the conditioner "just this once". Chloramine burns gills immediately.
- Cold water straight from the tap in winter. A 5°C drop can trigger a white spot outbreak within days.
- Only topping up evaporated water. Evaporation removes water, not waste, so topping up concentrates nitrate and hardness instead of diluting it.
- Changing water and cleaning the filter on the same day in a young tank.
- Blasting the substrate with the new water and clouding the whole tank.
- Using a bucket that has held cleaning products. Trace surfactant is lethal.
Should You Ever Skip a Water Change?
Only in two situations. During a medication course where the label says not to (and then you change water at the end of the course). And when a tank is mid-cycle with no fish in it, where you are deliberately letting ammonia feed the bacteria colony. Otherwise, the answer is no, and "the water looks clear" is not a reason. Nitrate, hardness and dissolved organics are all invisible.
Making It Faster
- A python style hose that connects to a tap lets you drain and refill without carrying buckets. Worth it for tanks over 3 feet.
- Keep two dedicated buckets so you can prepare new water while draining old.
- Pre-dose the conditioner into the bucket, not the tank, so the water is safe before it reaches the fish.
- Store a jug of aged, conditioned water for small top-ups mid-week.
- Do it on a fixed day. Sunday morning water change is a habit that outlasts motivation.