Forget about trying to glue two pieces of liner together while it's in the pond, it just isn't going to work. I don't know how big the pond is but from what it looks like in the picture I wouldn't go more than 2 Koi. Plant some cover so the fish can hide and keep out of the sun. Also, try putting fishing line about 8 to 10 inches above the shoreline. When the heron feel it on their legs they don't like to go any closer. But if they are landing in the water that won't help. A net of lead might be your only choice.