I use a pair of BRS 1.1ml/min dosing pumps using digital lamp timers to set duration and frequency for both my DT and FT. Ca doses at midnight/2/4/6 and Alk doses at 1/3/5/7AM so that there is plenty of time between the two to avoid precipitation.
For a 20g tank you could probably just manually dose although that can become a PIA. Really, unless it is just chock full of SPS/LPS, you can probably stay on top of things with just regular water changes using a good reef salt.