Two-Part Dosing Guide for Reef Tanks: Calcium and Alkalinity Balance

Maintaining balanced calcium and alkalinity is the single most important chemistry task in reef keeping, and two-part dosing is the most accessible method to achieve it. Whether you are growing your first frag of Acropora or managing a mature SPS-dominant system, understanding the principles behind this two part dosing guide reef approach will keep your corals calcifying steadily. This guide from Gensou Aquascaping at 5 Everton Park, Singapore explains the method from first principles through to advanced troubleshooting.

The Chemistry Behind Two-Part Dosing

Stony corals build their skeletons by extracting calcium ions and carbonate from the surrounding water. This process consumes calcium and alkalinity in a roughly fixed ratio: for every 20 ppm of calcium used, approximately 1 dKH of alkalinity is depleted. Two-part dosing replaces both elements using separate concentrated solutions. Part A is typically calcium chloride dissolved in RODI water, providing calcium ions. Part B is sodium bicarbonate or soda ash dissolved in RODI water, replenishing carbonate alkalinity. The two must never be mixed together before dosing, as they precipitate into insoluble calcium carbonate.

Target Parameters

Aim for calcium between 400 and 450 ppm, alkalinity between 7.5 and 9.5 dKH, and magnesium between 1280 and 1400 ppm. These ranges support robust coral growth while leaving a buffer against minor fluctuations. Stability within a chosen target matters more than hitting a specific number. A tank stable at 420 ppm calcium and 8 dKH alkalinity will grow better coral than one swinging between 400 and 460 ppm due to irregular dosing. Test both parameters at the same time each day to establish your tank’s daily consumption rate.

Mixing Your Own Solutions

Commercial two-part solutions work well but cost significantly more than mixing from dry chemicals. To make Part A, dissolve 70 grams of calcium chloride dihydrate (CaCl2.2H2O) per litre of RODI water. For Part B, dissolve 56 grams of soda ash (Na2CO3) per litre, or 84 grams of baking soda (NaHCO3) if you prefer a gentler pH impact. Soda ash raises pH more aggressively than baking soda, so choose based on your system’s pH tendencies. Both dry chemicals are available at hardware stores and online on Shopee for under $10 SGD per kilogram.

Determining Your Daily Dose

Test calcium and alkalinity at the same time on two consecutive days without dosing. The difference tells you your tank’s daily consumption. For example, if calcium drops from 430 to 415 ppm overnight, your tank consumes 15 ppm daily. Using the recipe above, each millilitre of Part A raises calcium by approximately 3.5 ppm in 100 litres of water. So a 300-litre tank consuming 15 ppm daily needs roughly 13 ml of Part A. Dose an equal volume of Part B. Adjust weekly based on ongoing test results.

Dosing Schedule and Method

Split the total daily dose across multiple additions throughout the day. A dosing pump programmed to deliver equal portions every hour provides the smoothest parameter curve. If dosing manually, divide into at least four doses spread across the day. Never add Part A and Part B simultaneously in the same location. Dose them at opposite ends of the sump or at least 15 minutes apart to prevent localised precipitation. Automated dosing pumps eliminate both the timing burden and the risk of forgetting a dose.

The Magnesium Connection

Magnesium plays a supporting role by preventing calcium and alkalinity from precipitating out of solution prematurely. If magnesium drops below 1250 ppm, you may find it impossible to raise calcium and alkalinity simultaneously, as they precipitate into calcium carbonate snowstorms. Test magnesium weekly and supplement with magnesium chloride and magnesium sulphate as needed. Most reefers add a third dosing channel for magnesium, though its consumption rate is typically only 10-20 per cent of calcium’s rate.

Common Problems and Solutions

Alkalinity dropping faster than calcium usually indicates heavy coral calcification, which is a good sign. Increase Part B slightly while keeping Part A constant, then retest after 48 hours. Calcium dropping faster suggests precipitation occurring somewhere in the system, often in areas of high pH around kalkwasser reactors or near CO2 scrubbers. White deposits on pump intakes or overflow teeth confirm precipitation. If both parameters resist rising despite increased dosing, check magnesium first. Low magnesium is the most common cause of stubborn calcium and alkalinity issues.

When to Graduate Beyond Two-Part

Two-part dosing scales effectively up to approximately 100-150 ml per element daily. Beyond that volume, the accumulation of sodium and chloride ions from the solutions can shift ionic balance over months, potentially affecting sensitive organisms. Large water changes of 15-20 per cent weekly counteract this ion buildup. For tanks consistently exceeding 150 ml daily, consider transitioning to a calcium reactor, the Balling method with additional trace supplementation, or a hybrid approach using both reactor and dosing pump. Regardless of method, regular ICP-OES testing every three months helps you monitor long-term ion balance and trace element levels.

Related Reading

• Balling Method Explained: Three-Part Reef Dosing for Stable Chemistry
• Reef Trace Elements Guide: Iodine, Strontium, Magnesium and More
• Calcium Reactor vs Two-Part Dosing: Which Is Right for Your Reef
• How to Set Up Auto Dosing for Your Planted Aquarium
• EI Dosing Complete Guide: Estimative Index for Planted Tanks