While my plans come together ill explain the whole system and how it will work. Most importantly how im going to use reef-pi to control it.
My system will be laid out like this:
5 lb co2 bottle: co2 supply
Vts250c regulator: regulates co2 pressure
Solenoid: turns co2 on and off
Needle valve: adjusts co2 amount injected
Flowmeter: using this instead of a bubble counter to see how much co2 im injecting
Check valve: prevent aqauriumnwater from backing up into the co2 system when co2 is off
Reactor: diffuses co2 into the water.
My plan is to use macros in reef pi and set my co2 such that it can bring the while tank up to 20ppm in 2 hours before lights on. Then let the ph controller maintain that level until about an hour before lights off. Then let the reactor run for another hour or two to clear any co2 that still may be in there.
The standard way to measure how much you are injecting is with a bubble counter. But with a 75 gallon, who can count 5 to 10 bubbles per second? Plus bubble counters are very imprecise, because every one has different orifices and make different sized bubbles.
For this reason im using a flowmeter. There is a thread on the planted tank forums explaining which ones measure very low flows that we are using in aquariums. Bettatail on that forum has tested and has calibration charts for some of these. Normally they are hundreds of dollars but not many sellers know the true value and they can be had for a steal with careful research.
Here is the one i found on ebay.