Hi,
Yes, evidently the Red Sea unit get good reviews though I haven't seen it for sale here in the UK. The fact is though that no matter how good your diffuser unit is, the CO2 laden water still has to be distributed quickly and evenly into the tank. The larger the tank the more difficult it is to achieve even and high saturation of the water column simply due to the volume, so what is as important as your diffuser is water throughput and circulation patterns in the tank. This means that your filter and/or auxiliary pumping will need to meet or exceed the 10X tank volume per hour rule of thumb. Big tanks suffer more than most from poor circulation due to frictional forces and momentum loss. The deeper the tank the more chronic this problem becomes. If your priority will be carpet plants then the throughput and circulation issue has even more relevance. Carpet plants are the least forgiving of poor CO2 and they wither away without the slightest hesitation. Carpet plants operate at a disadvantage because they are low and are generally the furthest away from the source of CO2 injection. Their proximity to the substrate means that the hydraulic flow rates are at their lowest and that detritus and decaying particles tend to settle in this region and so they have the toughest time obtaining nutrients/CO2 while being typically nearest to the bulk of organic waste produced in the tank.
For a tank of this size I would very much agree regarding the addition of a second diffuser, the effluent of each to be located at opposite ends of the tank. In this way the distance that CO2 laden water has to travel is less and the concentration more evenly distributed.
Inline systems can and do work well, so don't ignore their potential. I use inline diffusers in a large tank and I have no difficulty growing carpet plants - however they are part of a
system that focuses on high, stable injection rates, high water throughput and careful attention to circulation patterns which takes full advantage of the flow rates. These factors have an identical impact regardless of any diffusion technique.
A pH controller can work and many use it successfully, but the controller cares more about maintaining a stable pH as opposed to a stable CO2 concentration, so we tend to regard the controller more as a fail safe and monitoring device while using a timer controlled shutoff solenoid to regulate injection and a dropchecker filled with 4 dkh water as the main concentration measurement device.
Cheers,