Im going to put 2 of my marble angels "Long fin red eyes" and "wimpy"
into a 29 gal and really nothing else because they are medium size now.
There wont be room for anything else except maybee some small corys.
Getting an algae eater in your 20 will probably be overstocking it because
your angels will grow quite large in a short period of time.. Unless you have other tanks or plan on getting more tanks in the future to allocate your fish id skip getting an algae eater... just do regular clean up with
water changes and gravel vacuming, wipe the algea with a clean cloth during this period and you should be ok. What ever you decide to do I
wish you the best of luck with your tank and fishies