Why Is My Alkalinity Dropping?

The usual cause: your reef is growing

Stony corals, clams and coralline algae turn calcium and alkalinity into calcium carbonate skeleton. A growing tank uses more each month. Your water changes may have kept up at first, but as the corals grow, the drop between changes gets bigger.

Calcium is used at the same time, in a fixed ratio. For every 1 dKH of alkalinity used for growth, about 7 mg/L of calcium goes with it. If alkalinity is falling but calcium hasn’t moved at all, something other than growth may be involved.

Measure your daily use

Test alkalinity at the same time each day for a few days, without dosing anything new. Take the first reading away from the last and divide by the number of days. That’s your daily use in dKH.

For example: 8.4 dKH on Monday and 7.5 dKH on Thursday is 0.9 dKH over 3 days, or 0.3 dKH a day. In a 200 L tank, replacing 0.3 dKH a day takes about 1.8 g of baking soda a day. The dosing calculator does this for your tank.

Made-up readings to show the maths.

Other causes worth ruling out

If the numbers don’t add up, check these before you start dosing more.

• A test problem. An out-of-date reagent or a slightly different routine can move a result by more than your tank’s daily use. Retest, and compare with a second kit or a reading from your local fish shop.
• Precipitation. At high alkalinity or pH, calcium carbonate can form on heaters, pumps and in reactors. Look for white crust. It takes alkalinity and calcium out together.
• Nitrification in a new or heavily fed tank. Bacteria that turn ammonia into nitrate produce acid, which uses up alkalinity. This matters most in new tanks and those with high bioload.
• A dose that’s stopped. Check that the dosing pump is running, the tube isn’t blocked and the container isn’t empty.
• Fewer or smaller water changes. If you’ve been changing less water, less alkalinity is being put back.

Replace what’s used, little and often

Once you know your daily use, add that amount back each day rather than making a big correction once a week. A daily dose keeps the level flat. Weekly top-ups make it rise and fall in a sawtooth.

Bring a low reading back up gradually first. Bulk Reef Supply suggests no more than 1.4 dKH a day and watching that pH doesn’t move more than 0.2. Then switch to the maintenance dose. If you need help choosing a method, our guides to whether you need to dose and two-part dosing cover the options.

Keep checking as the tank grows

Daily use creeps up as corals grow, so a dose that’s right today will be slightly low in a few months. Test a couple of times a week while you settle on a dose, then weekly. Log each reading so you can see the trend before it becomes a problem.

How much alkalinity should a reef tank use per day?

It depends entirely on what’s growing. A soft-coral tank may use almost nothing; a tank full of fast-growing stony corals may use more than 1 dKH a day. Measure yours.

Can I just do bigger water changes?

Water changes put some back, at the level your salt mixes to. They work for low-use tanks. Once use outpaces your changes, dosing is usually needed.

Why did my alkalinity drop after a water change?

Your new saltwater may mix to a lower alkalinity than your tank. Test a mixed batch so you know what it adds.