Seachem Reef Advantage Strontium 600 gr.

Reef Advantage Strontium™ restores and maintains strontium to levels found in natural sea water.
Strontium is utilized by corals in the same manner as calcium and holds a similar importance in maintaining a vigorous level of growth. It is most useful in densely populated aquariums requiring a continuous supply of strontium.
Reef Advantage Strontium™ is a highly concentrated (30,000 ppm), ionically balanced strontium supplement that can be added directly to the tank (no predissolving required).
600 g treats 20,000 L (5,000 gallons).

Directions:
BEGINNER: Use a level teaspoon per 150 L (40 gallons) twice a week. May be added directly to tank.
ADVANCED: Check strontium level, then follow dosing regimen above until strontium is adjusted to 8–12 mg/L. Each dose will raise strontium by about 2 mg/L. Size or frequency of dose can be adjusted. Thereafter, use as required to maintain strontium.
EXPERT: After determining the strontium depletion rate, set up a continuous drip system. Use the following formula to determine how much to add to your top-off water: t=0.02vs (t=teaspoons to add to top off water, v=volume of tank in gallons, s=amount to raise strontium in mg/L). For example, if you want to raise strontium by 8 mg/L in a 50 gallon tank, then you would add 0.02x50x8=8 teaspoons into the top-off water.
HINTS: Use the Reef Status:™ Strontium™ to check strontium levels.