Salinity
The other readings shift with salinity, so get it right before you judge them.
- Typical target: about 1.025-1.026 SG (35 ppt)
- Check the refractometer with calibration fluid
- Top off with fresh RO/DI water only
Reef chemistry
Reef Dosing Basics
Corals use alkalinity, calcium and magnesium as they grow, so those levels slowly fall. Learn why they drop, how to measure what your tank uses, and how to replace it slowly. Test first, change slowly, and keep alkalinity steady.
What you are replacing
The big three, plus salinity
Stony corals, clams and coralline algae build skeletons and shells from these. A mostly soft coral tank may stay balanced with water changes; a growing stony coral tank usually will not. Ranges are typical targets, not rules.
Alkalinity
Corals use it to build skeleton, and it helps hold pH steady. It is usually the first number to fall.
- Typical target: about 7-11 dKH
- Pick a number in range and hold it there
- Change it less than about 1 dKH a day
Calcium
Corals take calcium and alkalinity from the water together, so they tend to fall together.
- Typical target: about 380-450 ppm
- Check it when alkalinity keeps dropping
- Raise it gradually over several days
Magnesium
Magnesium helps keep calcium and alkalinity in solution. When it runs low, the other two are hard to hold.
- Typical target: about 1250-1400 ppm
- Test every week or two while dosing
- Correct it first if it is low
Test first
Measure before you dose
Dosing replaces what your tank actually uses. You only know that number by testing.
Get a baseline
Test salinity, alkalinity, calcium and magnesium on the same day.
- Make sure test kits are in date
- Fix salinity first if it is off
- Write every result down
Measure daily use
Test alkalinity at the same time each day for several days, with no dosing or water changes.
- The average daily drop is what your tank uses
- Stop early if it nears the low end of range
- Check calcium at the start and end
Start a little low
Use the product label to work out what replaces that daily drop, then start slightly below it.
- Follow the label for amounts, never a guess
- Dose each part separately into high flow
- Split doses through the day when you can
Retest and adjust
Test alkalinity every day or two at first, then a few times a week once it holds steady.
- Adjust in small steps
- Expect use to rise as corals grow
- Retest after changing salt mix
Methods
Ways to keep up
Start with the simplest method that holds your numbers steady. Move up only when testing shows you need to.
Water changes
Fresh salt mix contains all three. For soft corals and a few stony corals, regular changes can be enough.
- Simple, and lowers nitrate too
- Test your mixed saltwater as well
- Falls behind as stony corals grow
Two-part by hand
One solution for alkalinity, one for calcium, added on a schedule. Magnesium is added as needed.
- Easy to start and adjust
- Never mix the parts together
- One big daily dose swings more
Dosing pumps
A dosing pump adds two-part in small amounts through the day, so levels stay steadier than hand dosing.
- Calibrate each pump head
- Label every line and container
- Smaller containers limit damage if a pump sticks on
Calcium reactors
CO2 slowly dissolves calcium carbonate media, adding alkalinity and calcium in a natural balance.
- Suits tanks full of stony corals
- Needs CO2, a regulator and pH monitoring
- Takes time and testing to tune
Common mistakes
Never dose blind
Supplements are concentrated. Too much, too fast or in the wrong spot can hurt corals more than a slightly low reading.
If the water turns milky after a dose, stop dosing, keep the flow running and retest before adding anything else.
- Dosing by the calendar, not by tests. Every tank uses a different amount.
- Fixing a low reading in one day. Fast alkalinity swings stress corals more than a low but steady number.
- Pouring both parts in one spot. They react and turn the water cloudy. Add each part separately into high flow, away from corals.
- Testing at random times. Alkalinity can shift through the day, so test at the same time.
- Ignoring magnesium. Low magnesium makes calcium and alkalinity hard to hold.
- Trusting one odd result. Retest and check the kit date before you act.