Diffusing CO2

In my opinion,

You should invest in a canister if you got pressurized, it'll just gas off your CO2.

If you are using Pressurized the best way is the REX style diffuser. It what I've used for about 4 months and I love it. Its cheap and super efficient. Also If you are interested in the seeing the CO2 in the tube it self just get some clear PVC.

REX reactor below.
http://www.rexgrigg.com/diy-reactor.htm

A glass diffuser should suit your needs just fine.They are cheap,and easy to hide,http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=9915

Cannister filters are not necessary for having pressurized CO2.It is just not recommended to diffuse it with a HOB filter.

i always suggest a reactor. that way you dont see bubbles

I do agree with Chonhzilla about the HOB gassing off ur tank but this isnt necessarily a bad thing. I have been running my 72 with prssurized since may and i feed the co2 through my canister, been working well so far. Last night i actually put an order in for 2 koralia nanos and a hydor ario 4 with blue leds. With the way i have things setup, i have very little surface break which is both good and bad. bad because no goodo2 exhange but good for keeping the gas dissolved. Hence why right now i have an airstone kick on at night to oxygenate the tank. Believe it or not, my dissolved co2 content stays pretty consistant with the airstone on at night since i keep my co2 system on 24/7 with the belief that it will help keep things balanced and so far seems to be working quite well.

On a note with what u are doing, a canister would be beneficial in almost everyway over that of a HOB. I would try to diffuse co2 with the HOB on and see if you can get consistant readings. I debated on getting an AC PH with venturi but i opted to just get a glass diffuser from fosters and smith and the plan is to put the diffuser below the koralia circ pump and have that suck the co2 bubbles and blow them around the tank. I opted not to get a venturi because it was more money and i wanted to try this first.

I am also running pressurized into a 29g and my glass diffuser works just fine, reactors using a canister filter are more efficient, but it is not as big of a deal in a smaller tank.

Keep the water level up in the tank to help eliminate splash from the HOB return and its not that bad on out gassing. If it has bio wheels I'd remove them though. In a planted tank the extra bacteria isn't really needed from them anyway. Investing in a drop checker is a good idea either way.

Unless you can get the HOB to not create noticeable surface agitation, you're basically wasting CO2. The point of pressurized is to increase the concentration of CO2 in the tank beyond what is equilibrium...any gas off will just work towards the lower level.

If you get a powerhead,you can diffuse the CO2 through that.I don't think it will be necessary though.