The screw lid is the exact reason I stayed away from reef octopus.
@gwdemos runs a dual chamber 5000 on his 300g system and has all kinds of trouble. Especially with the lid binding up when it comes time to change media. The screw lid has a small o ring that can easily wear out (this is by design as an oring gets compressed by the lid tightining)vs a large rubber gasket where compression is spread out over a much larger piece of material. He has issues priming the reactor as well.
Do not buy the aquamaxx T series reactors. I used one for about a month before I got into a big debate with marinedepot and eventually returned it. It's difficult to bleed and regularly builds up undissolved co2 gas.
I run a PM marine large reactor (it's almost a carbon copy of the geo) they've stopped making them.
With your system (500 plus gallons) I'd do a 6 or 8 inch diameter reactor chamber with a 500gpg-ish recirc pump.
The aquarium plants carbon doser (co2 regulator) is almost a must. Do consider an adjustable lab grade (not a neptune dos) peristaltic pump (cole-palmer masterflex or watson marlow pumps are best) to run the system. Try to purchase a brushless motor pump as it will be much quieter than a brush driven pump.
At the very least I'd remove the gate valve and replace with a pinch valve for more stable effluent rate. Gate valves clog easily with the grit from dissolved aragonite media. If you go this route you can have success, I do, but I also purchased a flow meter so I can quickly see what the flow rate through the reactor is. Timing flow into a graduated cylinder is annoying.
Another thought to consider is running in tandem with kalkwasser to help minimize the ph lowering effects of a reactor. I am adding about 2 gallons of saturated kalk a day with my reactor and has allowed my ph to have around an 8 to 8.24 diurnal swing. It was 7.6 to 7.9 previous to kalk.
Hope that helps
