long story short. just make sure you do it right the first time for dirted tanks , dont hop through the procedures or youll tear down the tank like I did.
Can you please elaborate on what you mean by "do it right?"
Every substrate becomes inert after awhile. I would go with black diamond blasting sand and root tabs. Even if you have to poke a hole in your carpet to replace root tabs every few months, they are not large holes. You only need one every several inches too (in a grid). Doesn't seem like a big deal to me in the long run. You could even poke the O+ tabs themselves through the carpet in a finer pattern. The capsules are only there so the O+ doesn't break down as you place it into the substrate. With a 75g tank, I don't think the minor deterioration of the O+ will be a big deal. I have had no issues with O+ bits resurfacing in my 75g and throwing off parameters. I'm not one for expensive substrates. My swords, crypts and sag have all grown fine in inert substrate with root tabs. I haven't replaced my tabs in 6 months and the growth seems fine.
As for the light, I think people tend to exaggerate the light needed. With medium light, which the Ray2 should give you no problem, and CO2, which you have, you won't have any issues. You should be able to grow just about anything. The 18" front to back may be a little too much for a single light. If you want a secondary fixture, I suggest a planted+ 24/7. The automatic sunrise/sunset would be nice and it would definitely push you into medium-high lighting. It would help with a more even lighting and give you some more colors than the Ray2.
Just another opinion.
It's funny that you mentioned just pushing tabs through the carpet. Maybe you're right and it's not a big deal.
I'm really grateful to be getting so much feedback. It definitely makes me more comfortable in making a good decision because everyone seems to use different methods and they all work fairly well for them...I presume...or they wouldn't be recommending it.
Now, I am going for a nature aquascape. I want hills, and therefore a substrate that is going to work with me on that. I was planning on filling tupperware with gravel as a base to help with that, but I'd like a substrate that will hold it's position as well as possible.
Honestly I've dropped a descent amount of money into this already, so saving some money on substrate that I can spend on more plants and, if necessary, fertilizer would be a huge plus for me.
How does black diamond blasting sand hold up for scaping?
If I were to use MGO, what cap would be best for holding hills?