7mag,

I'm no expert but you'll have to read the publication at the link. I do remember seeing you can buffer the potential PH drop with the gypsum.

I know this works as it has been done in a public lake in my state.

I can't believe you can't find any alum. Most water treatment plants use it and it's usually easily available from any local chemical company. The best part is it's not expensive.

Are you sure you're not over aerating and just recycling nutrients throughout you pond? If my pond was only 4 feet deep I'm not sure I'd even bother with aeration. Do you have a another nutrient source as in farm run off or something?

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.