
I found a $35 device that sticks on the bottom of the tank with magnets. You can adjust the settings for the height of the tank. There's an app for your phone and here's the readout after ten nights:

Here's the link to it:
https://www.amazon.com/gp/product/B01C5RQJHS/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Tony