hmm,micro swords should be alright,but they require constant pruning because from my experience,they can really grow~! since your tank is a 20gal,you may want to opt for shorter plants as your foreground to give the tank a greater depth. Even for my 100gal,i will be planning to use HC as my foreground. Or perhaps like what some of the others have suggested,you may wanna try riccia? The pearling effect can bring out a whole new dimension to your tank. But the disadvantage of riccia is that it requires quite abit of maintenance as the larger clumps tend to float up easily. Hairgrass can be quite alright also but i have observed them to be rather dark coloured at times so it might be tricky to find a balance in your aquascape.