That's a good question which I was going to ask something similar to regarding the chemicals. Reading the contents for the calcium bottle I notice it has strontium in it as well as magnesium and many other contents, which is more than I care to post on here at this moment. I'm using B-Ionic only because I bought the supplements for a reasonable price at a local auction. Others will chime in about dosing with kalkwasser, or dowflake and baking soda. I've heard in conversation at a LFS that strontium is not a key ingredient, same as trace elements only because the manufacturers are only pitching a business strategy to sell what you don't need. Been said all thats needed, calcium, alkalinity and magnesium which can be made at home and dosed into your tank with good success. Oh I add iodine when needed.
Proportions are 1ml to 4 gallons of water.
I don't find strontium or iodine to be key ingredients. If you really must know test your water. I find the trace amounts found in esv's two part is usually enough to maintain the life in most aquariums. But if you really must know you should test the water there's no other way to truly know.
Keep in mind every tank is different so one person may be using strontium then another person may not and they both may have beautiful tanks. There no real must in this hobby except that you must have some kind of container that holds water and water for the animals to live (but I'm sure someone will prove me wrong on that as well), all the equipment and additives are up for debate.