Ha GSPs are always delinquents, what a chap I know who used to work as a dog behaviourist called "permanently young at heart" My guess is that he has just realised that they are there. It took my boys (GSP x weimaraner)until they were about 2 and a half to realise that there were squirrels in trees, now one of the favourite things is to chase the squirrel from tree to tree around the garden and stand under the last tree with the mouth open, hoping for squirrel to fall off.
If you want to manage the situation, it wont be quick, .have a the dog on a lead and some of the food that your dog thinks is the yummiest in the world. start with the dog on the lead, way away from the pond, maybe even at the garden door and speak his name, when he looks at you praise and treat. If he doesn't then start the exercise indoors. once he always looks at you when you speak to him, move nearer the pond. The aim is to make you more rewarding than the pond. once you are at the pond and he will still look to you start moving backwards away from him so he has to move away from the pond to get the treat. Once he can do it the length of his lead, add something to the lead to make it longer and continue. When you are up to a decent distance. Start again from indoors or at the door with the lead trailing so you can stop him but don't hold it. When that is perfect take the lead off. Me I would never trust him though and would fence off the pond (mine is fenced off) The training will just break the obsession.