you can order spirulina flakes. mine also "battle" clown loaches and others for algae tabs. i've fed them boiled romaine, and i'd imagine they'd pick at cucumber, as well.
my understanding that dropsy in particular is a condition caused by internal bacterial infections, which are caused by diseases, not feeding in particular. gouramis do need a varied diet; however...in addition to a good staple food, supplementing with the above recommendations and live/frozen/freezedried foods is always a good idea.