I think it is best to place your diffuser after the pond is full.
Mount your diffuser on a disk shaped snow saucer or used, well worn, car tire. Both when tied to a rope, can be pulled into the pond from shore. They are like a sled and slide easily across the bottom. I can send you pictures if necessary.
Why are you not stocking LM bass in the pond?. Bass will do a better job than cats at controlling the bgill. cats will eat bgill, but I don't think they are as efficient at eating bgill as bass. I could be wrong. Want big bgill and no overcrowding problem?; stock only male bgill.
Fishing will be more exciting with bass than w/ cats. Northern bass will grow faster than the cats since cats do better in warmer waters. I tend to be biased for bass vs cats. Most all "nothern hatchery cats for sale" are raised in AK and south. Cats with no bass present might spawn and you could be overrun with little catfish. I would rather have the problem of too many bass rather than too many catfish. Abundant catfish will tend to make the water cloudy esp when they are spawning size.
