Mini CO2 reactor in an Aquaclear HOB filter v1.3b (shown on a Fluval Edge)
I think this has been discussed in a few threads but not really documented clearly, so I figured a DIY write-up could be helpful. I'm currently using this method to inject CO2 on a Fluval Edge (which comes with an AC20 / Mini HOB), but it should work on any AquaClear HOB. In contrast to bubbling the CO2 directly into the filter intake, this configuration keeps the airline out of the tank and avoids impeller gurgling noises.
Inside the filter, there is an impeller cover. Pull this out, noting the hole that is used to expel air when priming the filter.
The plan is to insert airline through the hole in the impeller cover and attach a diffuser to the other end. I used a broken chopstick piece. Another alternative is the Lee's discard-a-stone "slim", seen here. The space is pretty constrained so options are limited.
The airline and diffuser fit through the impeller cover like this:
Now, reinsert the impeller cover. You want the diffuser to stick out like this...it injects into a narrow column of water between the filter basket and the impeller motor housing
Note: the hole that we plugged up with airline tubing was there so air could escape while priming the filter. So we need a different way to expel the air trapped below the impeller cover and prime the filter. Solution: pull the filter basket tab in this direction while the filter is priming:
Its like a mini-CO2 reactor. Bubbles get jostled around the downflow and anything that is undissolved is trapped below the sponge. At least for me, the CO2 diffuses at a sufficient rate that a big CO2 bubble doesn't form.
Was looking for ways to improve the diffusion rate of the system without upping the bubble count so decided to give the ADA "ball glass" diffuser a try.
This piece is different from other ADA diffusers in that the bubbles are not emitted directionally but rather radiate uniformly from a point source. For this purpose it fits perfectly: its a nudge over 4" long (long enough to stick out of the top of the filter) and the ball is maybe 3/8 in diameter (small enough to fit under the impeller cover).
Essentially, we're just replacing the chopstick from the prior post with the ball glass. However, the diameter of the glass stem on the Ball Glass diffuser is larger than the hole we were using before. So it needs to be drilled / reamed out. Since I was getting out the drill anyway, I decided to relocate the hole to increase the water contact time. Here you can see the hole I drilled near the lower left edge of the impeller cover. Drill a small hole then test fit the glass stem and drill again (I actually drilled the hole a tad too large). If you don't have a drill I'm pretty sure you could ream this out with an Exacto knife as the plastic is very soft.
This is what the impeller cover looks like with the ball glass inserted:
Does it work any better? Well, my HC pearls more consistently which is positive but the finer bubbles produced by the ceramic diffuser find their way through the filter more easily too (ie contributing to some fizz in the water).
Going to try to run more concrete tests (probably time versus pH) over the long weekend comparing the two diffusers.