Hello there. I used to run a koi farm and herons were a constant menace until we set up a perimeter of motion-activated sprayers. The one we used was tradenamed "Scarecrow" and ran off batteries and hose power. Nets would sometimes work, but the herons would often sit on them and then poke their beak thru the now-submerged netting to spear the fish. Decoys worked for a few days but had to be moved frequently to keep the birds from catching on. Hiding places and steep sides on ponds help a lot, and sometimes are a complete solution,but I've seen a heron fish all night on a full moon, and eventually the fish got brave or stupid and came out. The sprayers are basically just the kind of irrigation device you find on a golf course that sprays water into the air in an arc combined with a motion sensor. You can set them up so that they will only trigger when something lands in the pond (not you when you go out to feed the fish) and there is a sensitivity switch on the head also, so that you can walk up behind, turn them off and then take your daily swim undisturbed (haha). Either way, I feel your pain! Good luck !
