How about leave the Oct. POTM up until after the holidays and start a new contest in January. Otherwise, any saltwater reef topic would be fine. Would not recommend a non saltwater reef topic to be posted on the front page of a reef club site.
Future topics:
Best fish; best invert (crab, snails, serp. stars, etc); best LPS; best SPS; best mixed reef; best sump; best macro algae picture; best full tank shot; best equipment room; best large fish picture; best deep sand bed; best bare bottom (reef tank that is); best frag tank set up; skimmer contest; best natural reef (snorkel/scuba); best ugly fish; best brown/ugly coral; best ricordia; best big stag pic; Best sea fan/gorgonian; best sponge picture, best shark picture, best tropical beach picture, etc. You could get specific, like best monti, best brain coral; best aussie acan or best angel fish picture as well. So many reef related topics to select from.