My pond pump runs for 20mins then cuts out?
Most pond pumps have a safety mechanism that turns them off when they get too hot. That is probably what is happening to yours. It means that your pump's life is ending. Might as well get a new one now. If your pump is under warranty, you can get it replaced for free. When you buy a pump, consider the length of warranty as part of the purchase price. More expensive pumps usually have a longer warranty and it can be worth it to buy a more expensive one.