Unless you have a very cheap light, like a FishNeedIt light, you have way too much light on the 20 gallon tank. Even one T5HO bulb is too much unless it is about 22 inches from the substrate (very good reflectors) or about 16 inches from the substrate (Fair reflectors). A standard 20 gallon tank is about 16 inches high, so the one bulb light would need to be about as far above the top of the tank as the thickness of substrate you use, if it has only fair reflectors.
When you have way too much light it is hard to provide enough CO2 to support the fast growth of the plants, and you need to provide plenty of NPK and trace elements for the same reason. If you don't, only the more vigorous plants will grow well, leaving the other plants starved for nutrients. This can quickly lead to algae attacks.
As you discovered, low light tanks can give much more satisfying results.