Seachem Reef Carbonate 500 ml.

Reef Carbonate™ is a concentrated (4,000 meq/L) optimized blend of carbonate and bicarbonate salts designed to restore and maintain alkalinity in the reef aquarium.
Calcium and carbonates are essential to all coral growth. If either becomes deficient, coral growth will cease, followed by a rapid decline in coral health. To prevent this you must provide carbonates (Reef Carbonate™) and calcium (Reef Advantage Calcium™ or Reef Complete™).
Used as directed, Reef Carbonate™ will not deplete calcium, magnesium, or strontium which usually tend to precipitate with increasing alkalinity. Reef alkalinity should be maintained at 3–5 meq/L (8–14 dKH). Alkalinity should not be allowed to fall below 2 meq/L.

Directions:
BEGINNER: Use 1 capful (5 mL) per 80 L (20 gallons) twice a week. Best if diluted into a cup of freshwater, then added. Check alkalinity every 2 weeks and adjust dose or frequency accordingly.
ADVANCED: Check alkalinity, then follow dosing regimen above until alkalinity is adjusted to 4–5 meq/L. Each dose will raise alkalinity by about 0.25 meq/L. Size or frequency of dose can be adjusted, but do not exceed 1 meq/L per day. Thereafter, use as required to maintain alkalinity.
EXPERT: After determining the alkalinity consumption rate, set up a continuous drip system. Use the following formula to determine how much to add to your top-off water: m=va (m=mL of product to add to top off water, v=volume of tank in gallons, a=amount to raise alkalinity in meq/L). For example, if you want to raise alkalinity by 1 meq/L in a 50 gallon tank, then you would add 50x1=50 mL into the top-off water.
HINTS: Best to pour in gradually near strong water movement. Do not add to a sump or other area of slow water exchange. Use MultiTest:™ Marine pH & Alkalinity to measure total alkalinity or Reef Status™: Magnesium, Carbonate & Borate to measure carbonate alkalinity.