Re: looking to start 150gal terrarium and need major advice.
Cant inject CO2 when you have animals in there. A 150 would be very difficult to catch the frogs... so you could CO2 after planting but before frogs/fish are added.
You would also have to change the water after CO2 before frogs/fish go in. I'm considering doing that on my new tank to treat the plants, but I already seeded pretty heavily with springtails, and of course the co2 would kill EVERYTHING but plants, even my seeded springtails.
A 150 with a water feature should stay pretty humid without much issue. Look for a poll on fans, you'll see a lot of people use recirc and venting fans in the tanks. A 150 could use a computer fan as a vent running 15 minutes every 1.5-2 hours easy without dropping the humidity too much.