Rex type reactor orientation

You *can* run it horizontally, but you'll have gas build up and it's definitely not the most efficient way.

If you have installed a venturi loop or some other way to burp out the gas it'll be fine. It still won't maximize the efficiency of the reactor.

Best to run it vertically.

Will installing it at an angle hurt anything? Hope to post some pics once I'm finished.

Well, that depends on the angle. The water should be flowing in the top of the reactor and out the bottom. It doesn't really matter where the gas enters. I suppose it's "better" if the gas enters from the bottom of the reactor, but even if it doesn't it'll be fine. What happens is, the gas gets trapped in the reactor and dissolved.

You can have it at an angle, but 45 degrees or higher from the ground for the most efficiency.

Vertical or angled is fine with the flow discharge at the bottom. These downflow reactors will not work well laying horizontal. I'm running three of them with two vertically mounted to the stand frame and one at about a 30 degree angle. All work great with no bubbling out the discharge.

The reactors work by having the CO2 bubbles fight against water current increasing their contact. The bubbles want to rise while the water is forcing them down. A perfectly tuned reactor would have the bubbles rising at the same rate as the flowing water, effectively holding the bubble stationary allowing it to completely dissolve.

If you ran it horizontal, the CO2 bubbles would simply be flushed right out the reactor since the bubbles wouldn't be rising against the flow. Slanting it will still work, but you still want to aim for the ideal rise to flow rate. Slant it away from 90 and you reduce the rise of the bubble so if you have low flow you may want to slant it.

Thanks for the information. An Eheim 2028 will be driving the reactor. Once I get it up and running (hopefully I can find the 5/8 barbs tonight) I will post some pics of my install.

Is any diffuser required if the reactor is vertical?

There shouldn't be any need for a diffuser if the reactor is doing its job properly.