Filstar XP2

Hi, I have a Filstar XP2. I was looking through this page here:

http://www.planetrena.com/Filstar-Rena-Parts.html

Specifically, I was looking under "Filstar XP inlet/outlet accessories". The part I am interested in is Part D, which connects to the spray bar (I believe it is the output regulator). (Unfortunately, I cannot click on the picture as it goes to a non-working page: http://www.planet-rena.com/html/parts.html)

My question is, there is a hole w/an airline tube coming out of the top. Besides regulating flow, what else is the function of the airline tube? Could I, say, plug in an outside input (like a DIY CO2 bubble maker) into this so that it exits along with the filter output?

Any reply is appreciated.
A: Thanks so much for the info on the broken link. The venturi feature on the flow control was discontinued many years ago and the veturi hole was capped. If you want to use that feature you could remove the cap and indeed pump air into that hole through tubing. There is a very weak venturi flow if you just want some extra aeration. But the weak flow is why the feature was discontinued. The flow controls sold today have this hole plugged so there is no cap necessary. You can buy parts at the store web site http://www.PlanetRenaDirect.com.