We have a moderately successful family fish tank: The fish seem to survive, the plants do not. (Also we’re really good at growing algae, and may start feeding it to the children.) With a coding problem, you Google it and get several excellent solutions. With a fish tank problem, you Google it and get several excellent solutions that contradict each other. So the excellent solution we’ve chosen to make the plants happy is to add carbon dioxide to the tank. Plants need it, and one of my favorite in-store tanks uses it. So it's settled. I thought I’d need to pick up a heavy tank of CO2, like when I rented a tank of helium. Turns out you can coax yeast to make it for you. This Instructable describes how, and is what I used to make ours. Step 1: The HardwareThe generator would be a bottle, a cork, an aquarium one-way check valve (to keep water from backing up into the bottle) and some tubing. We drilled a hole in the cork and super-glued the valve into place. (The wider part of the valve with the white stripe goes closest to the cap.) Then we hooked up a long tube. Apparently the trick with CO2 is that you need it to dissolve in the fish tank water, not just bubble up and out. So we positioned the output end of the hose right next to the intake valve of our aquarium filter. That way the bubbles will get gobbled up by the filter pump and spit back out into the tank as tiny bubbles. Step 2: The SoftwareAgain, we followed the Instructable, adding in some protein powder as mentioned in this different approach (natch). My favorite part of the directions is, “really any sugar will do but the white stuff is the cheapest, and we’re feeding yeast here.” And we ended up with a bottle of Yeasty Mix. Which we stoppered with our special cork and hooked up the tube. It is entirely possible we didn’t mix the yeast right. But we’ll watch it overnight and see how things go. If we get no bubbles, we’ll mix it up again. And I’ll report back on the plant situation.