These shells are great for adding hardness into your water and work as dechlorinator as well! Often times we need more minerals for our snails, shrimp, plants, goldfish and african cichlids tanks. What size you need depends on your aquarium. We recommend testing your water and adjusting based on water change schedule.
Here are some very loose guidelines:
3 Pak - 1 shell per month for 5 to 10 gallon aquariums.
2/5 Ounce - 1 shell per month for 20-30 gallon aquariums.
1 1/3 Ounce - 1 shell per month for 40 to 55 gallon aquariums.
2 1/4 Ounce - 1 shell per month for 75 and larger aquariums.