I have an android phone and about a month or two ago downloaded "Aquarium Note", I thought I would share because it's such a good app to keep on top of the different tanks you have. It's really simple and easy to use.
Some of the features include keeping a log of your fish as well as their date of birth/purchase and other aquatic animals and plants. You can record the setup date of your aquariums and there is an activity page for each tank that you have on there where you can record any changes you have made, or even just to remember when you last did a water change or medicated the tank. You can calculate expenses of aquarium, livestock and goods which are filtered by types and belonging aquarium.
The bit I like the most is being able to track water parameters (pH, temperature, nitrate, salinity, alkalinity etc) with a list and graph, I always used to write them down however the paper would be scooped up and thrown out so no real way of keeping track of it all in the same place. There is even a to-do list and push notifications if you set one as a reminder for a certain date/time.
Anyway I'll stop rambling on about it now, if anyone else tries it out let me know what you think!