Planaria can eat small fish eggs like those off zebra danios. One way to avoid this is take a small piece of chicken liver and place it on a saucer on the bottom near whatever area is giving cover for the planaria, then place several bits of gravel on the saucer, cover the saucer with a larger saucer that is upside down, the gravel has to be large enough that there is a open space btween the edges of the saucers. Do this just before lights out. discard the liver in the morning, it should be covered in planaria. repeat htis every night until you are not catching planaria or the eggs hatch.