Frogs!

[Danienov]

Some may think this is ridiculous but I think I'm going to get one for the sump cabinet just in case

Check out home.woot.com

Leak alarms!

 

[ganjero]

Not ridiculous at all, leak alarms have been use in the hobby for a long time.
Neptune systems sells a module for their controller http://www.neptunesystems.com/products/expansion-modules/leak-detection-ald/

There are DIY plans to add one to your controllers, etc.

Those frogs are very affordable, good find.