You can make one that is sensitive enough to add 10 Ml of water to a 20 gallon tank for under 20.00 bucks....I have a few that I have been using for over ten years with no problem....It is a radio shack black project box 2.39,a float switch 7.00,2 120v indicator lights one red one green,1.99,an on/off switch 1.50,power recepticle 1.89 and a power chord 1.59......Their is a picture of what one looks like on my page....