Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
When you run a bare airline into any kind of powerhead/filter, here is what frequently happens. The CO2 approaches the end of the airline. When the CO2 reaches it, instead of one bubble coming out cleanly, the vacuum from the filter suddenly sucks a lot of bubbles out at once, resulting in the end of the airline being filled with water again, sometimes up to multiple inches. Cycle repeats.
These bursts of multiple bubbles all at once are more than an impeller can efficiently break up, and results in large bubbles coming out that just float up and out of your tank.
Terminating the end of the airline with an airstone, even a very poor one, stops this vacuum effect from happening and greatly improves diffusion.
BTW, the AC110 and all Aquaclear HOBs actually make great diffusers if you set them up right.
First, cut the end of the airline not straight as usual, but at a long tapering angle; so that the end is about 1" long. Use multiple zip ties and secure the airline to the intake, so that the taper is pressed against the slots of the intake. This too eliminates most of the vacuum effect, and requires no modification to the intake.
Second, take a look at how water flows from the left (impeller) chamber of the AC110 to the right (media and outflow) chamber. Notice how the pump attempts to drive water sideways into the filter sponge at the bottom of the chambers.
The problem with this is that CO2 bubbles tend to strike the sponge without entering it, then float up and out of the left chamber.
The solution is easy. The filter basket is designed to accommodate two sponges. Mount a single sponge where the 2nd (top) sponge would go, and leave the bottom area open. Now the CO2 bubbles end up trapped, floating on the bottom of the sponge, with water churning and rushing past them to help them dissolve; an ideal situation. A few fine bubbles will manage to work their way through the sponge before being dissolved, and will be driven into your aquarium, where they'll still dissolve quite a bit before floating out.
Of course all HOBs agitate the surface of the water, which causes some dissolved CO2 to be lost to the surrounding air. Keep the water level as high as possible to minimize this, and you may find that your HOB works well enough that you don't need anything else.