Should you change all the aquarium water? No, and here is why
Never change all the water in your aquarium: that big clean-out destroys part of the bacterial balance, shocks the fish through the shift in parameters and often triggers a mini-cycle with a nitrite spike. Best practice fits in one sentence: 10 to 25 % of the volume every week, with water close in temperature and conditioned.
Why is dirty water actually precious?
Amber-tinted but balanced water beats crystal-clear new water: a mature tank hosts nitrifying bacteria on the substrate, decor and filter media, and the water itself carries stable parameters the fish are acclimatised to. Replacing everything means imposing a new pH, a new hardness and residual chlorine all at once — major stress, sometimes fatal to sensitive species. Clarity says nothing about quality: only tests speak, like everything to do with water maintenance.
How do you do a water change by the book?
- Frequency: 10 to 25 % per week, more if nitrates exceed 30 mg/l.
- Siphoning: use the opportunity to vacuum waste off the substrate with a gravel cleaner (8 to 15 €).
- Temperature: within 2 °C of the tank’s.
- Conditioner: neutralise chlorine and metals (6 to 12 € a bottle, months of use).
- Filter: don’t clean it the same day, and never under tap water.
Are there cases where a big change is required?
Yes, three: acute pollution (an unnoticed corpse, household product, treatment overdose), detectable nitrites in a stocked tank, or nitrates out of control. Even then, proceed by 50 % at most, repeated at 24-hour intervals if needed, rather than a brutal 100 %. That drain-it-all reflex features prominently in our 10 first-aquarium mistakes.
What does a good water-change routine cost?
Almost nothing: a gravel cleaner, a dedicated bucket (never the household one), a conditioner — 20 to 30 € of equipment for years of use. Allow 15 minutes a week for a 100-litre tank. Conversely, a tank restarted after a total drain often costs a treatment, tests and sometimes fish, not to mention weeks of instability — the same mechanism as a tank started too fast, described in buying fish and tank the same day.
Frequently asked questions
I am starting over after a disease — drain everything this time?
Only for a confirmed persistent parasite with the tank already emptied of its inhabitants. Otherwise, treat in the tank, keep up partial changes and leave the filtration alive.
Can I use rainwater or bottled water?
Rainwater, yes if collected cleanly, blended with tap water to keep some minerals; bottled water costs too much and its mineral content varies.
Can a heavily planted tank space out its water changes?
Yes: a densely planted, lightly stocked tank can run on 10 % every fortnight, tests permitting. The plants consume part of the nitrates.
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.