Now, I'm no expert...but this is my opinion:
That large island in the center of the tank is casting a shadow on most of the water portion of the tank. You should be fine if you stick to low light plants, add a low dosage of a fert mix at water changes, and dose flourish excel daily. If you want to use CO2, it should be fine also...but probably not necessary. All you need for DIY is a 1-liter or 2-liter bottle w/ cap, some CO2 tubing, and a filter you can hook the output tube to. Heck, you could also try using an airstone at the output. In a low-tech tank, you don't necessarily have to be efficient with CO2 injection. A level around 10ppm is probably more than approprite. Its all an experiment when you first start--especially in an odd tank such as this. You'll be combating algae at first, trying to determine what fert regimen and CO2 levels complement your light level.
(experts, feel free to correct me when I'm wrong!)
If you plan on keeping amphibians in the tank, you may want to consider eliminating CO2 as an alternative altogether. Its not necessary, but some amphibians are very sensitive to the dissolved gas levels in water. I'm not sure of excel's effects on amphibians.