Aqua Glow-in-the-Dark Fish Tank SMALL Rubble 8oz ~ Glow Pebbles ~ Aquarium Gravel Each stone is unique in size - generally they will be .5"-1.5" These can glow for 8 hours or more when fully charged. These charge the fastest under black, UV or LED light, but will still charge under regular light bulbs Water, acid and alkali resistant - Salt Water tanks are fine Completely Safe & Non-Toxic