FastAquatics
Step-by-step how-to

How to do an aquarium water change (the right way)

Step-by-step guide to performing an aquarium water change. Tools, parameter matching, dechlorinator, vacuum technique, frequency.

Beginner7 steps30 min

What you'll need

Step-by-step

Step 1: Turn off equipmentUnplug heater, filter, and powerheads. Heaters can crack if exposed to air while hot. Filters can lose prime.
Step 2: Vacuum substrateInsert the gravel vacuum into the substrate. Pinch and release to siphon water + lift detritus. Move across the substrate, focusing on uneaten food + waste hot spots.
Step 3: Drain to target volumeFor weekly maintenance: 25-30% (10 gallons from a 40g tank). For nitrate spikes: up to 50% maximum. Larger water changes need to be split over multiple days.
Step 4: Prepare replacement waterSaltwater: mix new RODI + salt to match display SG (1.025). Freshwater: dechlorinate tap water with Prime per dose instructions. Match temperature within 2°F using a thermometer.
Step 5: Refill the tankPour slowly to avoid disturbing aquascape. Use a clean pitcher or pump for large tanks. Saltwater systems benefit from filling with a powerhead running.
Step 6: Restart equipmentPlug heater + filter + powerheads back in. Verify filter primes (canister filters may need a manual prime). Watch for 5 minutes before walking away.
Step 7: Test parametersAfter 1 hour of equipment running, test ammonia + nitrite + nitrate. Verify they match expected post-water-change levels (lower nitrate by ~25-30%).

Frequently asked questions

How often should I do a water change?

Weekly 25-30% is standard for most tanks. Reef tanks: 10-15% weekly. Shrimp tanks: 10% weekly with carefully parameter-matched water. Always test rather than relying on a calendar.

Can I do too much of a water change?

Yes - changes >50% can shock livestock from sudden parameter shift. Split larger nitrate corrections over 2-3 changes spaced 2-3 days apart.

Do I need to dechlorinate water?

Yes for tap water (Prime, AmQuel+, or similar). RODI + saltwater mix does not need additional dechlorinator if the source RODI is clean.

Related guides + tools

Browse all how-to guides, calculators, setup guides, or the disease database for related help.

Aquarium-keeping fundamentals

Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.

Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.

Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.

Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.

Species-appropriate husbandry: research adult size, territoriality, diet, and tankmate compatibility before purchase. Use our tank stocking calculator + compatibility guides.

Frequently asked questions

How long does an aquarium take to set up? 4-6 weeks for full cycling + first stocking. Use our cycle ETA calculator + how long does cycling take.

What's the best aquarium for beginners? 20-gallon long. Big enough for parameter stability, small enough for budget + space. See beginner picks.

How often should I do water changes? 25-30% weekly. See water change frequency Q&A + water change calculator.

Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.

Related resources

Saltwater livestock · Freshwater livestock · Coral catalog · Care library · Q&A library (222) · Glossary (127) · Disease database (50) · Calculators (29) · Interactive tools (7) · Husbandry deep-dives · DIY projects · State legality directory