Hey Jim,
Are you ready to be blown away? This could get ugly, but don't shoot me I'm just the messenger.
You're right about the T-section, it is just another source of potential leak. Additionally, you never get even gas flow between the two diffusers, one always has less resistance than the other. Despite all that I really think the headaches might be worth it. As you said, that mist in one corner has to be distributed over the whole tank. It's just really difficult on this size. I use this splitter which has never leaked (knocks on wood twice). It has hose clamp type fittings on the ends.
http://www.aquaessentials.co.uk/index.p ... ts_id=1254
If one of the diffusers hogs the gas you could install one of these "speed controllers" on that side -
http://www.aquaessentials.co.uk/index.p ... ts_id=1253 this has similar fittings called snap shut. I don't bother with this I just turn up the needle valve and blast more gas through and eventually, the slow side gets it's share of flow. I don't have super-mega-expensive fish though
If you want to just stick with a single diffuser you ought to at least consider using an inline type like the Cal Aqua on the link I gave. That way you are dispersing the dissolved gas with more force into the tank from the filter output.
Speaking of filter output, I did a test of one of my supposedly 1700LPH filters filled with biomedia. I recorded a real world output of 800LPH
This was an easy test to do, just lift the output pipe out of the water and stick it in a 1 liter jug (or any size jug as long as you know what it's volume is). Time how long it takes to fill the jug being sure not to spill any water. I'll bet your XP2 sitting down below, fighting a 3 foot head , filled with media delivers a real output of about 500LPH. The Juwel Jumbo probably does a lot better because it's at water level but those 9 liters of media will slow it down some I reckon. Are you sitting down? Because of the back pressure and friction from the media George and I figure tanks need 10X the tank volume as a filter rating. For your tank that means the optimum is filter rating of 4050 LPH.
so you're halfway there. The only way to make of for this is to add more CO2 than you would do normally. I know, I know, the cabinet won't hold a Fluval FX5 and your wife would probably have you sleeping on the sofa for the rest of you life if you exchanged your canister.
These numbers are just something to be aware of.
Check this out - For my 600L, between the two filters, I've got a total of about 50 liters of biomedia. I've got a combination of the "noodles" at the bottom, zeolite/carbon mixture, Purigen and the sintered glass balls. I can't prove it but I think this gives me more surface area than sponges for bacteria colony.
Cheers,