KALK can be added to RODI topoff water at the rate of 2 tsp per gallon. Some like to start at 1 tsp per gallon and adjust upward. Even if we decide to utilize other Calcium and Alkalinity supplementation methods, Kalk is a very good compliment to them. The negative is Kalk dust can be caustic with manufacturers recommending the use of gloves and face mask. Also Kalk can be dangerous to our aquarium if overdosed as it may raise the Ph of our system to high to fast. Kalk should only be used to maintain Ca and Kh levels. It should not be used to raise levels as the addition will cause a spike in Ph. Use two part solutions individually to bring numbers in line with target. Then go to daily use of Kalk for maintenance. Our Kalk is produced with the best quality material available. This is a half gallon container.