Well, since you asked . . .
Aquarium Note (free on Play Store)
I'm not in the habit of pimping phone apps, but I'm really impressed with this one. The more I use this app (not Aquarium Note 2) the more I like it. I use it to keep track of everything I do in my hobby.
I use it to journal and document my activities and observations associated with my tank and equipment.
I can create reminders of things I need to do.
It allows me to record my test results and observe trends in graphs over time. I can customize and/or add parameters as I want. I can set “normal” ranges for any parameters and those that go out of normal will appear in red, instantly alerting me.
I can record, edit, and track all of my expenses, even sort them by type of expense.
It includes a dosing calculator and other useful tools.
The best thing I like about it is I'm able to create lists of livestock, including how much I paid for them, type, a photo of my choosing, and extensive notes on each. I even include entries regarding hitchhikers/pests.
Now I have everything I need regarding my hobby at my fingertips. Love it. If you're looking for an easy-to-use app that is useful and free, try it out.