What type (like brand and even name) of the testing kit that you use for your tanks? Do you have a all in one kit, or do you a variety of multiple dependent on what you want to specifically test for?
Don’t they have those all in one test kits for both freshwater or saltwater tanks? I know what they do not contain super specific testing aspects but would give you the best bang for the buck according what API sells more of together.
I tend to just recommend and go with the overall master saltwater test kit unless you want to test specifically elements/chemicals. It tends to be a better bang for the buck overall, whereas buying them all individually would cost a lot more due to them needing to package it, etc.