In order to have any effect on plant growth, CO2 gas must be dissolved into the water of your aquarium. Just letting it bubble out of a tube from your DIY Yeast CO2 generator or CO2 tank will have little to no effect on CO2 levels in the tank, especially if you're using a yeast generator (see the DIY yeast CO2 recipe). This is because larger bubbles don't dissolve as quickly as smaller bubbles. So the ultimate goal of diffusing CO2 into an aquarium is to make the smallest bubbles possible, and them keep them from reaching the surface for as long as possible. You need a reactor, or a device that breaks up the large bubbles into smaller bubbles and mixes it with the aquarium water. Below are the most popular methods for diffusing CO2 in the aquarium:
Bell - (very low efficiency) About the most low-tech way of dissolving CO2 is by using a bell-shaped (or any shape as long as it holds gas) container turned upside-down in your tank. The CO2 tubing is placed underneath the bell and the CO2 bubbles rise up and collect inside the upside-down container. The CO2 dissolves passively and very very slowly. This will give you almost no CO2 concentration in your aquarium just because you can't run the CO2 faster than it dissolves and it dissolves so slowly, plants probably use up the CO2 as soon as it dissolves. One way to make it moderately more efficient is to aim a powerhead or filter exhaust across the bottom of the bell (the open end). This diffusor can be made quite easily at home.
Airstone - (very low efficiency) This is probably the second least efficient way of dissolving CO2 in your aquarium, but with a little ingenuity it can be made more effective. Try to get the airstones that create the finest bubbles. Limewood airstones are best, but they clog fairly quickly. These make fairly small bubbles, but to be really effective you have to keep the bubbles from racing up to the surface and escaping. Try to position the airstone as deep as possible. You can also put the airstone under the intake or exhaust of a filter or powerhead to trap the bubbles and send them with the current, thus keeping them in the water for longer and increasing the efficiency.
Glass Diffusor - (low to medium efficiency) One of the most popular methods due to its ease and low cost, it is slightly better than a regular airstone, although it is basically the same principle. A ceramic disk emits fine bubbles, finer than an airstone. However, this disk can collect dirt and debris and become clogged with algae, creating much larger bubbles and reducing its efficiency. A soak in bleach will help to clear the blockages. Again, try to position the glass diffusor as deep as possible in the tank. It can also be made much more efficient by placing it under the intake or exhaust for a filter or powerhead. Glass diffusors often need higher pressure to work and they may not work well or work at all with DIY yeast CO2 systems.
Bubble Ladder - (low to medium efficiency) This is another inexpensive, simple option which consists of a series of zig-zagged ramps or a twisted ramp placed on upright on the side of the aquarium. CO2 enters from the bottom and the bubble is forced to travel along these ramps to reach the surface. Meanwhile, the movement helps to dissolve the bubble. These work by extending the amount of time the bubble takes to reach the surface, thus giving it more time to dissolve. However, the bubble is rarely completely dissolved by the time it reaches the end of the ramp and is allowed to escape to the surface. The downside to these is that they are often big, bulky, and ugly and there is no real way to hide them.
Filter or Powerhead Intake - (medium to high efficiency) Perhaps the easiest high efficiency option is using a filter or powerhead as a reactor. All you need to do is put the CO2 tube into the intake of either a canister filter or powerhead. The motor will suck up the bubbles which will be either immediately pulverized by the impeller (powerhead) or drawn through the filter media, dissolving rapidly along the way due to the high flow, and then into the impeller (canister filter). Most ot the time when using a canister filter, you won't even see the bubbles coming out of the exhaust as almost all of the CO2 is dissolved. A powerhead is a little less efficient, as it just spews the fine bubbles into the tank and some of these rise to the surface and escape, but not before taken on a wild ride around your aquarium. Depending on the filter or powerhead, the CO2 bubbles can make a fairly loud ticking or whooshing sound as they are met by the impeller blades, and some aquarists may find this too annoying.
In-line Reactor/Counter-flow Reactor - (high to very high) These can be DIY or bought, but tend to be quite expensive to buy or labor intensive to make. However, they can't be beat in terms of efficiency if they are working correctly (another issue if you build it yourself). It's basically a chamber connected to the exhaust of a canister filter or powerhead placed vertically. The lower end of the chamber is often left open or covered with a sponge, unless it is an in-line design, where the bottom is connected to tubing that returns to the tank. The CO2 is released into the chamber, and as it tries to rise to the surface, it is pushed back by the flow of water. Even if it makes it to the top of the chamber, it has nowhere to go but back into the flow of water (the top of these chambers is often shaped like a funnel to force the bubbles back into the flow). To make it more effective, bio-balls or other oddly shaped filter media can be put inside to help break up the CO2 bubbles as they are tossed around. The biggest problem with these reactors is that since they don't have any effective means of breaking up the larger bubbles, if too much CO2 is pumped in and the flow isn't strong enough, the bubbles collect and form larger bubbles which dissolve much slower. So balancing the flow of the water and the flow of CO2 is critical to achieving optimal diffusion.
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