i agree on using an Aquaclear 50 hob on a 20 gallon tank. i always at the very least double the filtration 'rating' (and i use that term loosely! lol) for any tank.
for example:
my 2.5 gallon planted has an Elite Hush 5 hob on it (stock is one male betta, one cherry shrimp, and a few ramshorns)
my 20 gallon planted has an Aquaclear 50 hob on it (stock is 6 juvenile australian rainbows)
my 55 gallon planted has an Aquaclear 110 hob and a Penguin biowheel 350 hob on it (stocking is just under what i would consider heavy - community tank)
my other 55 gallon has a Rena XP2 and a XP4 canister on it (stock is a growing oscar and an almost full grown paratilapia polleni)
if you find the flow is too much when using a filter that is 'rated' for larger than the tank size, you can turn the flow down (if the filter model allows that capability), and you can also hang a piece of polyester batting over the waterfall area (if the filter is a hob). i've done both of these things on the Aquaclear 50 on the 20 gallon. i used a large-ish river stone to hold the batting in place, or else it will just swoosh into the tank from the water flow.
as for decorating the tank: find a nice piece of driftwood or other piece of 'hardscape' that you really like and build your tank's decor around that. it's easier to start with the big pieces and add smaller things that go with it. not sure if you want to go with live plants or fake, but live is WAY better/nicer looking.
