Along with the macro elements magnesium and calcium, strontium is an important component of corals and other invertebrates and needs to be supplied regularly. Recommended dosage Use the cap as a measure. Add once weekly one capful sera marin 5 for every 100 liters (26.4 US gal.) of aquarium water. sera marin 5 was developed along with sera marin 1 and 2. Strontium and Calcium are required in a certain ratio.