I would suggest an electronic load sensing ballast.
Your old ballast/starter doesn't work because it is fixed at 20W. An electronic ballast will sense the load & adjust accordingly, so you are not tied to the 18W in future. Plus the electronic ballast is smaller, lighter, and is quiet - no fluorescent hum, and possibly more energy efficient.
I have not seen electronic ballasts at HD (here, all they have are the heavy electro-magnetic ballasts - rapid start, but non-electronic.) You are better off going to a lighting parts store, they will be able to tell you what is needed. <Plus, electronic ballasts at my parts store is cheaper than HD .... at least in Edmonton.>
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.