That was very helpful...so one 24w t5ho bulb on my 20g is too much (right now I have on 6700 bulb and an Anatic bulb)? That I did not figure on. I will have to reevaluate.
Yes. Let me clarify by going about it logically. It makes sense that the light given off by a light source is brighter if you are closer to it, right? If you are standing a mile away from a light bulb, that light isn't going to help you much: there may as well be no light at all. If that light bulb is 2 inches from your face, the light will be blindingly bright. The same basic principle applies here in that the distance from the light matters. How this manifests itself in the aquarium is mostly in the height of your tank if you standing the light on top of your tank with legs. If you had a 24" tall tank and the mounting legs put the light 2" above the top of that (and let's say you also have 2" of substrate at the bottom), the light will be roughly 24" from the substrate. Now let's look at your 20G long tank. Those tanks are roughly 12" tall or something like that. Think about that. Using the same example of light being 2" above the tank and substrate being 2", that light is now only 12" away from the substrate, or TWICE as close as on the 24" tall tank! The same light on a 20G long is WAY brighter than on a taller tank. The height of the 20G long is really what is making one T5HO bulb on top of the tank unusable without CO2 injection: the tank is so short that the light is really strong! So while you might get away with 1 T5HO bulb with a 20" tall tank or higher and no CO2, the same can't be said for your 20G long. This can be overcome by raising the light to lower its intensity or obviously switching out the light for something less intense.
For reference: I'm using one T5HO bulb on legs on top of my tank, but mine is 20-21" high (a 55G). I also am running pressurized CO2 (was running DIY CO2 before and it was also fine: little to no algae growth) and one bulb is enough to generate good growth. I have the option of going to two bulbs here, which would be extremely high light, but I'm not sure I want to do that and deal with algae.
Just remember this: it's a game of getting the right light, CO2, and nutrients. The CO2 and nutrients can be like the engine and horsepower of a car while the light is the gas pedal. You can have a really powerful car (CO2 injection, fert dosing) and don't hit the gas hard (low light) and be fine, you will just go slowly (slower plant growth). You have the option of going really fast because of your powerful car though. On the flip side, if you have no CO2 injection, then your car isn't capable of going really fast. If you push the gas pedal hard (too much light) and the car isn't designed for that kind of speed, things will get out of whack; you will get lots of algae growth (I know I know, a car won't break if you floor the gas pedal but just go with me here). The best plan if you don't have a powerful car (no CO2 injection) is to just lightly dose ferts (what you are doing is fine) and ease up on the gas pedal so you don't push the engine too hard. Just remember that light is the key.
If you read all that, I hope it helped!