10years and didn’t know this!Out of your list I would get the Geo 612. If you have the room and want the more media storage. I would get the Geo 618. I have Geo 618 on my 255gallon Tank with 100 gallon sump. I get 6 months to 12+ months usage out of media. Depends on media I use and method of dissolving the media. Explained below.
I have had my Geo for like 15 years or so....very long time. I'm using the original eheim pump recirculation pump it came with. Its a tank. Once I learned that the old school, set the reactor ph to 6.5 and roll with a drip rate wasnt the best way to go and moved to a dual stage regulator where I changed the worker pressure to 15psi. The bubble size was really small then and I opened up my effluent to nice stream and just control the ph in the reactor to what my corals are consuming. start hight like 7.5 or 7.6 and work your way down to what tank is cosuming. Someone posted a video of it one time and I tried it out years ago and never looked back. Much better that you never have to worry about calcium clogging up a slow drip rate. This is when my media started to last allot longer.
You will love the Geo if you decide to go with it.
How much is a nice flow of effluent?
How did you know what the corals are consuming? Measure the calcium and/or alk?