Minimal number of clicks/taps until I can enter a test result into the correct field of a new entry /the same entry as the different test I entered just before. Voice controlled would be great if it is reliable with the numbers or maybe asks you to confirm if a number is way off /outside your usual range. Integrated timers in the test entry menu for when you are running multiple tests simultaneously.
Regarding lifestock management, I don't know whether it would just be a gimmick, I guess that depends on how well it works, but an overlayed camera preview so you can line up the same camera angle for growth pics of coral over time, idk if that can be done at the OS level to enable people to use the camera app they prefer.
Also with lifestock management, a button that makes a new entry for a frag or multiple frags that copies all the info of the entry for the mother colony and immediately asks you whether you want to take/select images for it, and automatically keeping track of the relationship between frags and mother colonies
Integrate with all the autotesters, most have some kind of API
Secure cloud storage of all the data, accessible via browser, particularly on PCs
Nice looking, but primarily functional graphs of the test data, both in the app and the website
Share buttons for graphs or test entries that provide nicely formatted pictures
Just some notes, suggestions, and gripes I have with a similar (paid) app
Edit: oh and integrated salinity/waterchange/parameter change calculators are quite handy, couldn't do without those