OK, because there are some strict chemistry things (like analyzing organics in the aquarium), but that's probably not what you'd want.
There are tons of questions, and some will be much harder nuts to crack than others.
For example, in ultra low nutrient systems, some SPS corals are said to get "burnt tips" when the alkalinity is higher than 7-8 dKH (say, at 11 dKH), while those same corals in higher nutrient systems seem not to suffer that way. First off, is it a phosphate or nitrogen effect, and beyond that, why does it happen?
I expect I know the answer to this one, but is any strontium needed in the water for hard corals to thrive (I expect not). Many reefers seem to think it is useful, but the number of folks thinking so has declined over time. A DIY salt could be made with and without strontium, and you could compare coral frags from the same parent in side by side aquaria.
That same format could be used to test a lot of hypothesis (iodine, phosphate, nitrate, etc. ).