
I have been keeping planted tanks for a long time. I recently set up a DIY CO2 generator for my tank and had used Fluval's diffuser in the past in a similar setup. I ordered another for my latest planted tank and the bubble counter for ease of use.
You do not need to have commercial tanks of CO2, a fancy regulator, or an expensive setup in order to have lush CO2-assisted growth. I use an old Gatorade bottle, some black airline tubing, Fluval's diffuser, and this bubble counter. I wasn't going to get a bubble counter but it was so cheap on Amazon that I went ahead and ordered it, and I am glad that I did! It provides another piece of equipment that prevents the tank from leaking back out the tubing onto the floor and allows you to see if your generator is still producing gas.
For a DIY CO2 generator:
1. Gatorade or other thick plastic bottle. Many people recommend 2-Liters. I think these are too big.
2. Fleischmann's Active Yeast, or some other baker's yeast. Not the Fast Rise kind
3. Airline tubing. CO2 tubing is really expensive and since you are generating your own CO2 for pennies, don't worry too much about gas escaping.
4. Check valve. Very important.
5. Plastic inline connectors for tubing
6. Bubble Counter
7. Diffuser
8. Silicone (if needed)
Drill hole in lid of plastic bottle. Affix plastic inline connector INSIDE the bottle with one side pointing down into the bottle and the other pointing up out of the lid. (if necessary, you can use silicone to get an airtight seal) Attach a small length of airline tubing inside the bottle. This length should be short enough that it will not dip into the Yeast and sugar mixture in the bottle. Affix another piece of airline tubing to the connector poking out of the lid. This goes to the bubble counter. Place the check valve between the counter and the diffuser.
The bubble counter will allow you to count Bubbles Per Second for your tank. (Various plants require different levels of CO2 in the water and are almost always measured in this BPS count.) It should have about an inch of water in it and be attached to the exterior of the tank. As gas is pushed into the counter, you will see a bubble form from the stem inside. It will pop and the gas will float to the airspace above the water level, and after enough pressure is built up, will begin pushing gas into the tank via the diffuser.
For this bubble counter, make sure that the intake and output valves are screwed as tightly as possible. I found a gas leak on the intake because I didn't have the tubing pushed down far enough on the stem.
The diffuser should go in the tank in a location that will provide for the maximum transfer of CO2 gas with the water. In my tank, I have attached it directly underneath the intake for my Fluval 305 filter. The CO2 goes into the filter, slowing through the media for a longer time of gas exchange, then hits the impeller as it is pushed back into the tank. You do not want to put the diffuser near the surface or around an airstone. In fact, you don't want to use an airstone for oxygen in the tank during your photoperiod. Oxygen bubblers cause CO2 to not be in the tank long enough to have a good gas exchange. (I use an air pump at night to make sure that oxygen levels to not drop dangerously while the lights are off and photosynthesis has stopped for the day. I turn off the pump about an hour before the lights to come on to allow CO2 to build back up in the tank and be readily available to the plants when the lights come back on) If you do not wish to have your CO2 go through your filter, put it in an area as close to the bottom of the tank as possible, with as little water movement as possible. You can hide the diffuser anywhere if you are using a bubble counter because you will still be able to tell if the generator is producing gas by looking at the bubble counter.
In the bottle, mix warm water with sugar, swirl it around until the sugar dissolves, then add the baker's yeast. The amount of sugar and yeast that you use will determine how long the generator stays charged. I have had bottles last two weeks, and I have had bottles last two days. Basically, you want there to be enough sugar to feed the yeast as it reproduces. More sugar means longer CO2 production. Keep in mind that a whole packet of yeast will be waaaaaaaaaaaay too much and may cause your bottle to burst.
After putting the lid with its attached line back on the bottle, walk away. It takes time for the yeast to produce enough gas to provide sufficient pressure. You will be tempted to open it. Don't. If after three hours there are still no bubbles coming through the diffuser, one of four things has happened:
1. You have a leak in your system (fruit flies and/or the smell of rising bread will tell you this)
2. You did not use enough yeast, your yeast was not active, or your water was too cold
3. You are impatient and still need to wait
4. You have water in your system and you will have to wait for internal pressure inside your bottle to overcome the water pressure from your tank (this is why I recommend the check valve)

I love this thing. For the life of me, I just could not get my DIY bubble counter to completely seal. No matter what I did, it will spring a tiny little leak somewhere, thus preventing the necessary build-up of pressure. For the 5 bucks you'll spend on one this product, it's a no-brainer. It is way more attractive then an old jelly jar, bottle, or whatever you would plan to use as a bubble counter, it can adhere to the side of your tank with a clip or a suction-cup (both provided), and the aggravation you'll save yourself by not having to drill holes in a jar and seal it off with tons of silicon would alone make this product worth the money. It's small and can easily be tucked out of the way.
My setup:
DIY Co2
-2x 1 liter bottles
-Fluval Bubble Counter
-Feeding into a powerhead for diffusion
-1 teaspoon yeast, 2 cups sugar, 3 cups water mixture in each bottle
-About 2 bps
Buy Fluval 88g-CO2 Bubble Counter - 3.1 Ounces Now
The round shape of this bubble counter magnifies the bubbles a bit to make them easier to see. Both a clip and suction cup are included for mounting, but neither work well. The clip is too small the fit on anything but a rimless tank and the suction cup will not stay stuck. On the plus side, the clamping hose barbs fit nice and snug.
Overall this is a decent item for the price.
**Update**
The barbed fitting on the output had cracked causing my tank to empty itself over the weekend. I loosened the fitting to remove the hose and the barb snapped off completely. Changed my rating to one star and will be buying something else.
Read Best Reviews of Fluval 88g-CO2 Bubble Counter - 3.1 Ounces Here
Build quality is excellent, it comes with a suction cup to attach it to the aquarium. No leaks and it does what it supposed to do.
Want Fluval 88g-CO2 Bubble Counter - 3.1 Ounces Discount?
This bubble counter works well for a CO2 freshwater planted aquarium. Connecting it up is a simple matter, but mounting it takes a little ingenuity. Be sure not to connect it too close to your valve from the CO2 source, as should the water back-up too far your solenoid might get water inside, which could render it useless. Check valve(s) strategically placed will prevent this type of damage. It is made out of plastic. It's great for any size tank with a CO2 system, pressurized or not.