Water Chemistry & Cycling Calculators

Water quality is the hobby

Fish and plants live in the water, so almost everything in fishkeeping comes back to it. The tools on this page help you do the routine work: change water, size heaters and filters, dose conditioners and test-kit chemicals, and understand hardness. They do arithmetic, and the dose rate itself must come from your product's own label.

Which calculator answers which question

• How much water, and what will it do to a reading? The Water Change Calculator shows the gallons and the dilution effect.
• What heater? The Aquarium Heater Size Calculator works from volume and the temperature gap.
• How much filter flow? The Filter Flow Rate Calculator gives a turnover minimum.
• How much dechlorinator? The Dechlorinator Dosing Calculator uses your bottle's rate.
• How much ammonia to cycle? The Ammonia Dosing Calculator is for fishless cycling.
• How much aquarium salt? The Aquarium Salt Dosing Calculator covers freshwater salt.
• How much of any product? The Dose by Volume Calculator scales a label rate to your tank.
• GH or KH in degrees and ppm? The Hardness Converter converts between them.

Water changes and dilution

A water change dilutes what is in the tank with what is in the new water. If you replace a quarter of the water and the tap reading is zero, a nitrate reading falls to three quarters of what it was. One large change will not always fix a high reading, because the dilution depends on the percentage and on what is in your tap water, and regular, consistent changes matter more than the exact number. Match the temperature of new water and treat it for chlorine and chloramine before it goes in.

Heaters and filters

A heater has to make up the heat the tank loses to the room, so the right size depends on volume and on the gap between your target and the coldest the room gets, not the average. Larger tanks often do better with two smaller heaters than one large one, which gives a backup. Filter flow is described as turnover, the number of times an hour the tank's volume passes through the filter. Common minimums are about five times an hour for a lightly stocked freshwater tank and more for a heavily stocked tank or a reef, measured at the pump's rating, since real flow is lower once media and plumbing restrict it.

Cycling and dosing

A new tank needs time to grow the bacteria that convert ammonia to nitrite and then to nitrate. Fishless cycling uses pure ammonia, with no surfactants, detergents or scents, so confirm purity before you use it. Dosing tools turn a target concentration and a solution strength into a volume to add, and you should enter the strength printed on your bottle. Add gradually and test before adding more.

Hardness: GH and KH

General hardness (GH) is a measure of dissolved calcium and magnesium, and carbonate hardness (KH) measures the buffering that keeps pH stable. One degree of hardness is 17.848 ppm as calcium carbonate, which is the standard conversion. The converter switches between degrees and ppm so you can read a kit, a product label or a species guide that uses the other unit.

Common mistakes

• Dosing from memory. Always use the current label.
• Adding water straight from the tap. Treat for chlorine and chloramine first.
• Sizing the heater for an average room. Use the coldest night.
• Skipping testing. A kit tells you more than the fish's looks.