If you do have the white or yellow grubs that Bruce describes I can tell you that their life cycle involves snails, fish, and fish eating birds such as herons. Do you see herons, egrets, kingfishers hanging around your pond? The bird eats a fish with grubs, the grub eggs I believe live in the bird until it poops in your pond. The eggs hatch and somehow live in snails. Your fish eats a snail and the grub burrows into its flesh. Then it all starts over. I got some of the yellow grubs in my fish when I had herons hanging around. Break the cycle by getting rid of the snails or the herons and within a month or two no more grubs.
