A planted tank is easy without added CO2. Just look at my 10G guppy.
Remember fish exhale CO2.
I LOVE El Natural planted tanks with a soil substrate (covered with sand or gravel)
The bacteria in soil produce CO2, and break down fish waste. (To the point where one partial water change every 3-6 months is the norm!)
Plus you won't need liquid ferts, since the nutrients will be released from the soil.
Soil substrate tanks thrive with artificial light (about 1 wpg) alongside some natural sunlight from a window. My 5G is in a windowsill with 2wpg of light- not a speck of algae and the plants exploded. I keep having to pull plants out for my other tanks (which is a good thing!)
It's also the only tank where the cabomba grows well. Look into it. :)
My 10G guppy is just gravel, and the plants do OK. Nothing like the soil one though.
For a guppy heaven, I would use lots of:
"Guppy grass" (I forget what it actually is.)
did I mention that soil substrate tanks don't use a filter at all? The plants do ALL of the filtration.
Maybe that's like the "old days".
I'm in the process of converting all my tanks to soil.
Just buy some organic choice potting soil from wal-mart, and put two inches in the bottom of the tank. Cover it with gravel, and SLOWLY fill it up with a few inches of water.
Plant your plants, then fill up the rest of the way.
(I ran a filter on mine for a little while to get rid of the original "murkiness" from planting, and now it is just running with a heater and light. No filter at all.)
Might want to add some crushed coral or agricultural limestone to the soil to maintain a PH of around 7.5-8.
Also, for your guppy heaven, you should get some Endlers Livebearers.
If for no other reasons, they're colorful, don't eat their young, and are on their way to extinction in the wild.
they do interbreed with guppies though, so might have to do a devoted tank.
Lots of color variants... Here's just one.