As far as I know, fertilized redbelly eggs are orange(-browish) in colour, and are laid in a nest (well, pit) in the gravel, not attached to the glass.
And redbellies (I'm assuming you have natts...) are sexually mature when they reach a lenght of about 6". Are your fish (or at least one pair) darkened in coloration, and fiercely guarding one specific area in their tank? This is spawning behaviour.
One more question: do you have any other fish with your piranha's?