Posted: Wed Jun 20, 2012 5:48 pm Re: Moe's new 75G home - build thread.
The CO2 system I use is a 5# pressurized tank with the milwaukee regulator and the Red Sea 500 co2 reactor. There's pictures of it on previous pages in this thread.
To answer your questions, it depends on a few things. What type of plants? How much lighting? etc. Could you run a low tech tank without CO2... certainly but you'd generally need to supplement the water with liquid carbon like seachem flourish excel.
Plants will generally grow faster with co2 and ferts added.
The fish, shrimp, snails, etc would produce waste that helps to fertilize the plants. They probably won't produce enough co2 to benefit the plants.
Turtles at least in the case of mine would just eat the plants - co2 wouldn't be needed at all, neither would lighting or substrate. If I drop a plant in my turtle tank, it's usually gone within an hour.