I am guessing overflows can be different but with mine, which I consider fairly standard, I have had this happen a couple times and what I did and worked was turn off the return pump and wait for it to stop flowing then slowly pull out the return plumbing just enough to break the seal and the rest of the water would go down the drain. that left maybe 1/2" of water on the bottom and pull the plumbing right out as it should not be glued in. I was then able to reach in and get the fish. Piece of cake.