A T5HO single tube light will have to be about 24 inches above the substrate in order to do without CO2. If that is a 40 Breeder tank, it is 36L x 18D x 16H, and lighting an 18 inch front to back deep tank with a single tube light isn't the best idea, but with that light raised that high above the tank, it should work well enough. 20,000K is really more for reef tanks than for planted tanks, but it might look all right to you.
I think the tank is a bit small for Discus, but it should work for angel fish, until they grow too big.
If you decide to put dirt under the existing substrate, read the sticky in the substrate forum about mineralized topsoil first. This is the wrong time of year to try to mineralize topsoil in your state by the conventional method, but if you don't mind the smell, you can bake it in the oven to mineralize it. (That means to get rid of the organic nitrogen compounds in the soil, by oxidizing them, ending up with nitrates, which the plants need for food anyway.)
I think the tank is a bit small for Discus, but it should work for angel fish, until they grow too big.
If you decide to put dirt under the existing substrate, read the sticky in the substrate forum about mineralized topsoil first. This is the wrong time of year to try to mineralize topsoil in your state by the conventional method, but if you don't mind the smell, you can bake it in the oven to mineralize it. (That means to get rid of the organic nitrogen compounds in the soil, by oxidizing them, ending up with nitrates, which the plants need for food anyway.)