they're not harmful in small numbers. wash out the tank with a 5% bleach solution and then wash it out really well so all the bleach gets out. i would put your frog in a quarentine set up for a day or two while you fix the tank. keep it bone dry except for the waterdish. springtails can't live in dry conditions for long and they don't live in pure water. they are attracted to anythign rotting (plants, wood, poop ect) and also moist tanks. to prevent it i would just keep the tank cleaner and you shouldn't have a problem. certain species (dart frogs) will relish in this "problem" and eat the springtails. good luck and i'de be glad to help if you have any more questions.