Water Change & Dechlorinator Dose
How many gallons to swap at your chosen percentage, and how much dechlorinator to add for the new water.
Dechlorinator for the new water: about 0.5 ml
Dose is estimated at 0.1 ml per gallon of new water, a common starting concentration. Check your specific dechlorinator bottle's dosing instructions before adding it, since concentrations vary by brand.
How it works
Pick your tank size and how much of it you plan to swap out, and the calculator gives you two numbers: how many gallons to remove and refill, and roughly how much dechlorinator to add to the new water before it goes back in the tank.
A 20-gallon tank at a 25 percent change means pulling and replacing 5 gallons. At 0.1 ml of dechlorinator per gallon of new water, that works out to half a milliliter, which the calculator rounds up to a visible 0.5 ml minimum since most droppers can't measure anything smaller. Scale up to a 55-gallon tank at 30 percent and you are swapping 16.5 gallons, needing about 1.7 ml of conditioner.
Dechlorinator only needs to treat the water going in, not the whole tank, so the dose scales with the change volume rather than the tank's total size. Dosing the entire tank's worth every time wastes product and, with some formulations, can affect water chemistry more than necessary.
FAQ
How often should I do a water change?
Most established freshwater tanks do well with a 25 to 30 percent change weekly, though heavily stocked tanks or ones still cycling may need more frequent, smaller changes. Our weekly aquarium maintenance routine lays out where the water change fits alongside filter and glass cleaning.
Why does new water need dechlorinator at all?
Tap water almost always contains chlorine or chloramine added for human safety, both of which are toxic to fish and to the beneficial bacteria running your filter. See how to dechlorinate tap water for your aquarium for the full picture, including temperature matching.
Is the dosing rate the same for every product?
No. This tool assumes a common 0.1 ml per gallon starting point, but concentrated formulas can call for far less, and older or diluted ones can call for more. Always check your bottle's label; it is the only accurate source for your specific product.
Should I vacuum the substrate during a water change?
It is the easiest time to do it. Pulling water out through a gravel vacuum removes settled waste at the same time, so you get two jobs done in one sitting. Our gravel vacuuming guide covers technique for both bare-bottom and planted substrates.