In passive hydroponics, a gauge to check the level of water – or rather the nutrient solution – is essential.

Though it is very easy to build one yourself, you might who prefer a “real” gauge. There are quite inexpensive.

Here are some links to webshops where you can find water gauges for hydroponics.

Lechuza, one of the manufacturers, sells them directly online:

Some grow shops in Holland sell them cheaper:

And in the USA: