It sounds like you're pretty well set with for reefing with the gear you have. So you're now at the point where you can start adding luxury items to keep tank maintenance to a minimum.
An auto topoff system would be great.
Anything to balance and automate Ca/Alk would be a nice addtion. A calcium reactor or a kalkwasser reactor inline with your auto topoff system would be nice.
A controller would also be nice, but one wouldn't be fully utilized on a 20g tank like it would on a larger system.
And if you like to DIY, you could experiment with different types of wavemaking, like surge devices.
Or if you get really
[hr]
into DIY and automation, you could set up a phytoplankton reactor to pretty much automate feeding. Perform an internet search on "Geosapper" to get some more ideas on this.
-OR- You can skip most or all of those suggestions and just go hog wild adding corals.
