I also have a 20 gallon tank and yes, it's not the easiest to clean out but when I hear what people have to go thru with larger tanks I don't feel so bad.
I have way too many critters in my tank to empty out completely and algae is growing on sides and bottom of the tank. I use one of those glass aquarium sponges, it makes a mess in the water but if you do it before you take the water out its okay. Next, I use one of those inexpensive aquarium vaccuums. Basically a ciphon design. Its not the greatest but it works okay for me. I make sure I have a large bucket (5 gallon painters bucket) for the water and dump it outside when I'm done (biodegradable rationalization here). I can only get 2/3 of the water because I don't want all my critters to die. Then I just fill the tank back up with tap water (temperature controlled of course). It's long work and I do it usually twice a week. It really depends on the state of my turtle tank and the clearity of the water. I use a 40 gallon TopFin filter from Petsmart and change the filter every 2 weeks.
I don't have any gravel in the tank however I do have a few garden rocks, fake plants, anarchis plants, a "Turtle Dock" from Zoo Med, and a "tree stump with cave" in the tank. basically, I just inspect my tank daily when I feed my turtle.
Its not nearly as hard as one would think, just need an hour or so a week.