If you’re running a “planted” aquarium, CO2 is a must. This helps the plants thrive and out-compete nasty algae. In general it leads to healthier, more fuller plants. One inexpensive way to accomplish this is to create your own CO2 by fermenting sugar with yeast in a soda bottle.
The problem with that is it’s hard to control the output of the bottles. Turns out someone is selling an inexpensive DIY CO2 gauge and valve setup. It can be used with one or two bottles and costs less than $14.
There is one catch, the reason it’s so cheap is that it’s shipped directly from the factory in China. Therefore, it will take at least a couple weeks to arrive in the mail via free shipping. There are also express shipping options for an additional fee if you don’t want to wait.