All great questions, hopefully this reply helps you and any others who would read, as I've experimented with SO many different horizontal reactors and their setups...
- I will have a spare inline diffuser which I plan to use for the method of injecting the CO2 (obviously without the actual ceramic diffuser). Anyone else used this method? Any specific design tips on using that method. I'll be using a bypass for sure so I guess as long as its implemented after that.
Yes, it does work, but if your reactor isn't on a "bypass" arm in your flow circuit, then the full flow of water can make the bubbles really loud. Specifically, I used the
GLA inline diffuser with no ceramic part, just like your idea, but had to switch it out for a different solution because the rush of water carrying the bubbles was loud. Also, if your flow is high enough, the rush of water will atomize/dissolve/sweep away your incoming CO2 injection -- and carry it right out of your reactor's exit (if the flow through your reactor is too fast).
If you were to set up your filter system with a controllable bypass (
see this post here for more examples)
or
it probably wouldn't be an issue. However, if you will have decent flow through your reactor, using an inline piece before the reactor to inject CO2 might not work great because of the noise/atomization issue.
- I had planned to just use a standard PVC pipe. Is it worth being new to all this to obtain a clear pipe if I can to make it easier to see what's going on or is it not really necessary once you have it setup and have a bit of a handle on the flow?
I know this is a bit controversial, but I think everyone should consider a clear reactor. Especially if it's your first, it will help BIG TIME for identifying issues and sorting things out in the long run.
- Overflow mode - I assume if you have it sized right (I'll take @Naturescapes_Rocco suggestion of 15), that as long as you provide atleast enough co2 to cover that or above, it automatically goes into overflow mode? Do you then just adjust the CO2 based on the frequency of purges?
Yes, spot on. Once you get your overflow mode CO2
ppm at the desired level through the day, you can gently dial back your CO2 injection until it purges every few minutes or so, instead of every few seconds. But remember, CO2 is fairly cheap -- would you be willing to pay an "extra" $15 per month to have total piece of mind that you're injecting enough? If so, it's not such a bad thing to have your reactor purge a bit more frequently, ensuring that your CO2 stays solid and consistent.
Most regulators on the market actually have trash needle valves that prevent fine tuning, so many users will attempt to "reduce" their CO2 purging frequency, but accidentally dial it way down. The good news is, with horizontal regulators setup in overflow mode, you can't really "over" inject CO2 beyond what you have it rotated to. You could have it purging every 5 seconds, but if the purged bubbles aren't getting atomized or swept in the current (bonus points if you have a gentle flow that allows them to exit straight up to the surface) you cannot over-inject your tank. It's awesome.
- Flow intensity - One of the reasons for adding a bypass so I can control flow. The reactor will be used in conjunction with 2 x netlea G2 pre-filters and an jebao mdp 6500 pump so I can adjust the flow rate a fair bit. This may relate to my question before about having a clear pipe so i can see the flow. How do you initially setup the flow rate? Is there a rule of thumb or just trial and error?
I love that everyone is trying this setup out. I think it's the best! Been rocking it for 2 years now, and just setup my new 120cm tank with two prefilters and a pump, too.
If you REALLY want to optimize your CO2 setup, get a splitter and have your reactor on a totally different exit (meaning, have two lily pipes returning water back to your aquarium, like this
The lily pipe return without reactor is your "main" pipe for 75% of your filter system's flow; the ball valve on that line will let you fine tune how much flow goes into your reactor line instead.
By having this type of two-lily-pipe-return, you can be sure that your reactor's CO2 purging doesn't get atomized/dissolved/swept into the tank. It stays completely silent, never changes, and prevents over-gassing of your tank, too.
IF you wanted to have your water return to your tank via one lily pipe (boo!), it works, too:
But when the reactor's purged bubbles meet the 'main' water stream, they can often become atomized/dissolved/make noise. It still works fine (I had this setup for 2 years) but it's not the 100% ideal setup, IMO.
- Apologies if these questions are obvious. It will be a case of once you setup one and play around with it, it'll all seem straight forward.
This forum is exactly made for these kinds of questions. While the majority won't comment, your questions and our answers will be read by many many people, and hopefully make this hobby a little easier for them, too!
this pic is looking down on the reactor from above 
