you made the candles differently? Instead of a taper candle you made disc candles like the pictured floating pool candles (Lots to choose from - check any browser) and find out if they would float on the surface of the melted ice? . You might also consider freezing the water with a plexiglas clear or color cylinder "core" inserted. When the candle burns down then simply knock the core plug out and insert another. Probably not cheap, but might work.