Well after testing this design for the last month after final tweaking (learning from mistakes) I feel comfortable posting this. Plus it may spur other ideas on CO2 diffusion

I tried all kinds of methods for diffusing CO2 in my 150g tank. Misting wasn't an option since the soda bottle look wasn't pleasing to me. I tried DIY pvc units with different pumps and all of them used way to much CO2 and were not consistent enough to my likings.

So while wander Home Depot for ideas I came across GE's SmartWater filters and figured what the heck it might work and will give me extra water filtration. Thus making the diffuser a dual purpose unit.

I found that using to units inline with each other. 30 micron poly spun in the first, and 10-15 micron in the second worked quite well. Main problem was that as they clogged CO2 diffusion decresed due to the loss of flow. So I then tried a heat drilled .5 inch hole in the bottom of each filter hoping I wouldn't loose the filtration aspect. This worked fine for the 300gph pump. But I wanted to get rid of the powerheads from the tank, so I hooked up a Mag 9.5, and then found that a .25 inch hole worked much better to balance the CO2 diffusion and filtration to best levels.

I replace the 30 micron every three weeks, the 10-15 micron lasts four to six depending on how well I vacuum each week at water changes.

The added benefit is if you ever get cloudy water you can throw in a .5 micron normal or .5 micron carbon pleated filter in the last stage to quickly polish the water.

The replacement cartridges are relatively cheap and vary in price. Home Depot 2 packs for $7 - $10, Ace Hardware just about the same as Home Depot but I find their 30 micron is much thicker and lasts longer before the core begins to discolor, and WalMart $4 - $6 for 2 packs. I found the Walmart filters to last easily half as long before the cores show discoloration but they are a cheaper short term option.
Obviously the pump size, size of hole in filter, cleanliness of tank will all play a role in how long each filter lasts.

My current bubble rate is 6 bps to keep the tank at a constant green/yellow per the 4dkh drop checker. 4bps keeps the tank at a good forest green. But the fish show no signs of distress at the current levels.

This is a pic of the filters after three weeks of use

Also with this desing I get no gas build up in either chamber which is a plus.

I should also note this is not exactly a cheap setup. Each canister runs around $35. You can get them cheaper without the bypass valves or clear cylinders though.

Good food for thought. Thanks for posting this.

For me, the ultimate solution would be for canister filter manufacturers to automatically put needle wheel impellers in their filters and the gph rating should reflect the use of these impellers. At that point, you could either put the CO2 into the intake hose or into a barb on the filter built just for that purpose -- I actually like the idea of the CO2 running thru the intake hose, because I'm sure a lot of the CO2 would dissolve during the trip thru it. With the needle wheel, you could also probably put air in thru this way to easily oxygenate your water, which might come in handy at night.

I actually had good results by simply putting the CO2 into the intake grate of my filter -- I did not modify my filter's impeller to be a needle wheel, however, because my gph is already borderline low. Most of the CO2 completely dissolved, but in the afternoons CO2 would accumulate in the filter and that occasional release of bubbles was loud and was going to drive me crazy!

I've been using the "misting" technique, and I do not mind the bubbles, but I can understand that some would not like them. My bubbles are so small, however, that I cannot see them unless I stick my face right up to the glass -- maybe mine are smaller than yours were.

Anyway, good creative thinking on your part -- the extra filtering is really a nice bonus. I assume those filters are for drinking water ... I hope they are not pulling good things out of your water.

They're just polyester, same as filter floss.

Where does the CO2 enter the filter? This is basically a standard external reactor, split into two pieces, with filter elements in place of the bioballs. Right? This takes less room, vertically, under the tank, and adds more filtration, but with the disadvantage of requiring more maintenance.

The CO2 enters near the prefilter intake via a barb in the 3/4" tubing. Yes, essentially its the same concept as a standard external reactor.

The disadvantages are more of a advantage.

1. Easier to clean then an external reactor. Which you could see how quickly an external reactor with bio balls could become.
2. Less maintenence on the canister filter
3. Takes only a few minutes to replace filters
4. Can be used as a standalone water polisher for emergencies. 1 micron carbon pleated filter 2 packs are less than $10.

I believe the Marineland Magnum micron cartridge that people use to hold DE powder is something like 8 micron. I'm not sure how tightly the DE powder itself filters, but that suggests that you could use a whole-house sediment filter like this for a DIY diatomaceous earth filter for green water issues.

Absolutely. And replacement cartridges are significantly cheaper, and you have a wider variety of micron choices.

what was the initial cost for the unit? what sort of extra fittings were needed to hook it up inline? could they be used without filter media with bio balls as external reactors, advantage being not having to find expensive clear pvc pipe.? what part number are they?

Each 10" standard housing was around $30. The model I use is GE GXWH20F. I preferred the clear housing to be able to view when to change each cartridge. There is a 3/4" x 4" schedule 80 pipe connecting the two units. Both the input and output are schedule 80 3/4" threaded, 3/4" hose.

You can find the 10" cartridges on Ebay in bulk for $1 a piece. I change the cartridges every two weeks. You can probably find the housing cheaper on Ebay than what I initially paid also.

Due to the design I'm not sure you could use bio balls. Water comes in from the outer portion and is forced through the cartridge into the center and then out.