Dosing fundamentals: when to start, what to use, how to dial it in

For the first few months of a reef tank's life, weekly water changes do most of the work. The fresh saltwater replenishes alkalinity, calcium, magnesium, and trace elements faster than the small coral biomass can consume them. That stops being true around month 6–12. As corals grow, their daily consumption of alk and Ca climbs past what a 10% weekly change can supply — and you start watching alkalinity drift down between changes, sometimes by 0.5–1.0 dKH per week.

That's the signal: you need to dose.

This guide covers when to start, the three main dosing methods, and how to actually dial in a routine that holds parameters within reef-tank tolerances. The math is handled by BRS's official dosing calculator — they publish the verified per-product dose constants directly from the manufacturers. What you need here is the judgment layer underneath: when to dose, which method, and how to dial it in.

What you're actually replacing

Corals — and to a lesser extent coralline algae, snails, and clams — pull calcium carbonate (CaCO₃) out of the water column to build their skeletons. For every 1 ppm of calcium consumed, alkalinity drops by ~0.14 dKH in roughly the ratio they're needed. So alkalinity and calcium consumption move together. Magnesium gets consumed more slowly (months between meaningful drops) but holds the other two in solution — if Mg drops below ~1200 ppm, your alk and Ca dosing stops working efficiently.

The classical reef-chemistry targets:

• Alkalinity: 8–11 dKH. Most reefers settle in the 7.5–9.5 range. Stability matters more than the absolute number — pick a value and hold it within 0.5 dKH.
• Calcium: 400–450 ppm.
• Magnesium: 1300–1400 ppm.

If you're not familiar with what these are or why they matter, read water chemistry basics first.

When to start dosing

Test alkalinity weekly. As your tank matures, you'll see one of three patterns:

1. Alk stays flat between water changes. Your corals aren't consuming much yet. Keep doing water changes; no dosing needed.
2. Alk drops 0.3–0.5 dKH per week. Light dosing helps. Either bump your water change to 15–20% weekly, or start small daily doses.
3. Alk drops 0.8+ dKH per week. Water changes can't keep up. You need a dosing routine.

Don't start dosing because someone told you to. Dose because your tests say you need to. A tank with no SPS and three softies will never consume enough alk to require dosing — that owner just needs to keep doing water changes.

The three options

2-part (the default choice for most reefers)

A 2-part system is two separate bottles: Part 1 is sodium carbonate or bicarbonate (alkalinity), Part 2 is calcium chloride (calcium). You dose them at the same time, in the same volume, scaled to your tank's consumption.

Why it's the default:

• Independent control. You can adjust alk and calcium separately if one drifts faster than the other.
• Predictable. Brand dose charts are well-documented. Use a calculator, dose, retest in 12 hours, repeat.
• Cheap to start. A small Tunze, Bubble Magus, or Jebao dosing pump runs ~$60–150.
• Scales to any tank size. Works for a 20-gallon nano or a 300-gallon mixed reef.

Recommended brands (in rough order of how often they come up):

• BRS Pharma 2-Part — granular soda ash + calcium chloride. Cheapest per dKH dosed. Best value if you don't mind mixing it yourself in RO/DI.
• Red Sea Reef Foundation A+B+C — pre-mixed liquid. A bit more expensive but no mixing needed. The "C" bottle is magnesium, dosed less frequently.
• ESV B-Ionic — boutique pre-mixed liquid. Excellent quality control, popular with SPS-focused reefers.
• Tropic Marin Carbocalcium / Bio-Calcium — German pre-mix, premium price.

Once you pick a brand, plug your numbers into BRS's official two-part calculator — pick your specific product from the dropdown and it returns the dose in ml using the verified per-product constant.

Kalkwasser (saturated lime water)

Kalkwasser is a saturated solution of calcium hydroxide — Ca(OH)₂ — dissolved in RO/DI water. You dose it through your auto-top-off (ATO) reservoir, so every drop of evaporated water gets replaced with kalk solution, lifting alk and Ca in a balanced ratio.

Why some reefers love it:

• Self-balanced. Saturated kalk delivers ~1:35 ratio of alk-to-Ca by molar consumption — close to what corals use.
• Free pH boost. Kalk dosing pushes pH up, which corals love.
• Phosphate sink. High-pH kalk precipitates phosphate, modestly lowering nutrients.
• Cheap to run. A bag of Ca(OH)₂ powder is ~$15 and lasts a year on a mid-sized tank.

Why it's not for everyone:

• Capped by evap rate. You can only dose as much kalk per day as your tank evaporates. Small or covered tanks don't evaporate enough to deliver enough alk for heavy SPS consumption.
• High pH solution. Saturated kalkwasser is pH ~12.4. Plumbing has to be careful, and it can't share lines with anything acidic.
• Reacts with atmospheric CO₂. Open ATO reservoirs lose potency in 1–2 weeks. Seal yours.

Most reefers run kalk as part of a hybrid — kalk plus a small daily 2-part dose to fill the gap. Plug your tank size and evap into BRS's kalkwasser calculator to see what you can deliver.

All-in-one (the "I don't want to think about this" option)

Tropic Marin All-For-Reef and Red Sea NO3:PO4-X-style dosers package alk, Ca, Mg, and trace elements into a single bottle. You dose one volume daily; the formula handles the ratios.

Pros: Simplest possible routine. Cuts dosing pump channels in half. Good for low-bioload mixed reefs.

Cons: Less control. If your alk drifts low but Ca is fine, you can't adjust them independently. Higher cost per dKH dosed than DIY 2-part.

Who it suits: Reefers who want a "set it and forget it" routine, lower-demand mixed reefs, and tanks where the keeper is travel-heavy and needs simplicity over precision.

How to dial in a routine

The process is the same regardless of method:

1. Measure your consumption. Test alk in the morning. Don't dose anything for 24 hours. Test again the next morning. The drop is your daily alkalinity consumption in dKH.
2. Calculate the dose. Run your tank net volume, current alk, and target alk (current + the daily drop) through BRS's official dosing calculator. That's the daily volume.
3. Split the dose. Don't deliver a full daily dose all at once — split it across at least 2 doses for nanos, 4–6 for larger tanks. Spread doses through the day to minimize pH swings.
4. Test in 12 hours. Did alk hold steady? Adjust the daily volume up 5–10% if it dropped; down if it rose.
5. Repeat weekly. As corals grow, consumption climbs. Most healthy SPS tanks see consumption climb 10–20% per month for the first year.

Stability is the goal. A tank running at 7.5 dKH stable beats one bouncing between 8 and 9. SPS in particular notice 1+ dKH swings within 24 hours — that's the #1 cause of RTN (rapid tissue necrosis).

Common mistakes

1. Dosing without testing. "I dose 10 ml a day because that's what the calculator said" — but you never retested alk after a week. You're flying blind. Test weekly, minimum.
2. Dosing into low Mg. If magnesium has slipped below 1200 ppm, your alk and Ca will precipitate out of solution and clog your dosing line. Check Mg before starting; bring it to 1300–1400 with a one-shot dose if needed.
3. Dosing Part 1 and Part 2 too close together. Drop both into the same spot in the sump within seconds of each other and they can react locally, creating cloudy precipitate. Stagger by at least 5–10 minutes, or dose them into different sump chambers.
4. Underestimating consumption growth. Your dose at month 9 is not your dose at month 15. Recheck every 4–6 weeks; bump as needed.
5. Forgetting to top up the reservoir. Doser runs dry, dosing stops, alk drops 2 dKH over a weekend, SPS dies. Use a doser with a low-level alarm, or check it weekly.

A practical timeline

• Month 0–3: Cycling and stocking light. Do not dose. Weekly water changes cover everything.
• Month 3–6: Mostly softies and a few LPS. Probably still no dosing — water changes hold parameters.
• Month 6–9: Heavier LPS or first SPS. Start testing alk twice weekly. If consumption hits 0.5 dKH/week, start a small daily 2-part dose.
• Month 9–12: SPS growing. Daily dose is established. Test weekly. Consider adding kalkwasser to your ATO for pH and free Ca.
• Year 2+: Routine is set. Mostly automated. Spot-test weekly, adjust monthly.

For the actual numbers:

• BRS official dosing calculator — two-part alk & Ca, per-product constants verified
• BRS kalkwasser calculator — daily Ca(OH)₂ dose and resulting lift
• Tank net volume calculator — always feed net volume into BRS's dosing calc, not your tank's nameplate

If you're newer to chemistry, water chemistry basics covers what alk, Ca, and Mg actually do in the tank. Dosing is the next layer up: how to replace what your corals consume, in the right amounts, on a schedule that holds parameters tighter than water changes alone can manage.