Painted glassfish are small transparent fish that have been injected with dye and sold at pet stores. According to the Fish Channel, many "painted" fish are sold without the customer knowing that the color is artificial.
There are different ways to dye a fish, including injecting the fish with dye with a syringe, soaking the fish in dye and introducing the fish to large amounts of hormones. The process of "painting" a fish can cause the fish to die or have a shortened lifespan. While the physical damage to the fish is often permanent, the dye fades and eventually leaks out into the tank.