Just a thought, but perhaps you could remove the stand pipe all together and replace it with a removable trash guard. You could incrementally drain the pond with the gate valve.(I think "Fire Is Hot" eluded to this idea in his last post) I wouldn't think that it would take very long to drop the water level a significant amount via an 8 inch drain pipe.
As far as removing the stand pipe from the 90 ell. Good luck! I ran some large bore PVC pipe on a job some years ago and hope to never do it again. I recall separating stuck fittings by way of using a large heavy file or rasp. I would hold on to the end of the file and lay it perpendicular across the pipe about 2 foot away from the fitting, and then slide it with moderate force, repeatedly striking the protruding edge of the fitting, alternately moving around the circumference of the pipe to prevent the fitting from getting cockeyed, and binding. You could probably use a sharp cornered piece of flat bar or something similar in place of a file. In your case, you might try removing the 90 from the horizontal pipe first, with the standpipe still attached, and then remove the 90 from the standpipe. Try having someone rock the standpipe back and forth (not side to side) while you firmly but gently rap the throat of the 90 with a 4x4 piece of lumber. Or, if you have room, use the file/flat bar method. It also helps to squarely cut the pipe, and bevel the ends with a file or grinder, before assembly.
Good luck. Looks like an interesting project.
