If you opt for a sump on your 75g, I strongly suggest you get it drilled for a bean animal type of overflow (ghost overflow box). If I had to do it all over again that would be the number one thing I'd changed. Currently, I have a Herbie overflow method, while it's not as bad as a Durso it's still not dead silent like a bean animal. The other downside to the Herbie method is it requires very precise tuning, you have to get the ball valve just perfect. With the Bean Animal method, there are two proprietary drains and therefore much less finicky when tuning.Trying to saturate almost 200 gallons is way beyond my largest, but I know this has been discussed on Barr Report. Some of them switched to Cerges reactors while others added a second Griggs. The drop should be achievable with your starting pH though.
The big thing I noticed most of the guys with large tanks talked about was using a dedicated pump for the CO2 reactor rather than running it through a filter which could do down over time.
Is your sump sealed in any way? Some of them seem to do this while others don't. I'm hoping to try a sump when I get my 75. I have a lot of work to do on the house before I can do that though [emoji14]
I'm hoping to avoid having to purchase and plumb another pump. I don't really have the room under the stand for more electrical either. That will be the absolute last resort as that would require additional changes and overhaul to the plumbing.
I actually went and purchased supplies needed to downsize the output of the reactor down to 1/2" and 3/4". Right now I have the 1/2" already installed and it's working. PH is dropping down into ideal range. However, the flow has also dropped. I'm going to leave it for a day or two and then try the 3/4. I think when I switch to the 3/4 I may add an additional reactor I already have lying around. This would allow me to have higher flow into the display while also giving CO2 more contact time in the reactor to dissolve. Going to keep experimenting to find the best option.