I would expect that this would be enough light to keep most mosses alive, but I think you're right that it probably won't spread very fast. CO2
would be helpful, but probably not strictly necessary.
If you do decide you want to give CO2
a try, since you're dealing with a 75 gallon about the only thing I would recommend is pressurized CO2
. Any other CO2
options would either be too expensive or too difficult to get good steady CO2
levels. With only the moss your tank would much too susceptible to nutrient imbalances, and the moss could become quickly over run with algae if there are problems.
I'd definately suggest putting in some good fast growing stem plants while you get the aquarium established and are growing in the moss. This will help prevent other problems. After the aquarium has been running at least a couple of months and the moss has grown in to your likings, you could start slowly removing the stem plants.