Calcium question

[Maru777]

Hello everyone! I need some help please! Some of my catts were doing and not doing so well - they were growing like crazy but then the new growths kept on having issues I did not know what to do with. After some reading I came across an excellent article on calcium deficiencies and recognized my plants in the example pictures basically. So I clearly need to add calcium to my water (I should have thought of that since it has basically no calcium - as in, the last water report I saw had “values to low to measure” for calcium in a few of the months).

So after the boring background - the question. I could not find any calcium feed on my little island except one for tomatoes that says calcium nitrate 15.5-0-0. But I have no idea how to use it on my orchids since the instructions are for tomatoes - of which I have none those instructions are one cup to 50 ft of row - I don’t even understand what that means though I understand all the words thank you!