Hello to all the fellow pond owners out there!

First and foremost this is my first post to this site and would like to thank everyone in advance for any advice you can offer me.
I have two ponds fairly close together that where dug for their clay, sand, content to be used as structural fill for the homestead lot. They are both about 50' wide and 100' long ovals, approximately 7'-8' deep in the center. so my best guess considering the slope from 0 to center and curvature of the oval is that they are around 300,000 gallons each. They are surrounded by swamp land that was created when the beavers decided the dam up a small creek at the rear of the property. The ponds are about 15 years old now and nothing lives in them beside giant bull frogs and water bugs. The water to me looks and smells tannic. Visibility is around 1'-2' normally, and 3'-4' deep on a good day. The water feels slimy to touch, but leaves you skin feeling coated when let to air dry. I don't know what the PH is but I plan to test it next week. The water it self is feed by the ground water table, and rain.
My question is what would be the most cost effective way to clean these ponds up, so I can let the kids swim in them, and possibly stock them with Cat Fish or Koi for them to throw their poles at once in a while?

A few more details about the pond is that their is no permeant power out at there at the lot yet, and hoped not to have to install any for a while. All power is from two generators I have out there, some solar cells, and car batteries, connected to invertors. There is a 120' well on the property that if need be I can draw from. I have lots of 8" and 2" pipe. A garden hose sump pump, 1 1/2" grinder pump, 3" grinder pump, 50 gal sand filter. A few dozen plastic rain barrels, rain gutters, filter fabric, and lots of miscellaneous construction material let over from projects I've completed over the years.

I don't mind getting creative with what I got, but don't want to go at this in the wrong direction, throwing money into a literal hole in the water buying a bunch of chemicals I may or may not need. Any ideas you guys have would be greatly appreciated. I'm really looking for a long term solution here, something that is cost effective, and in harmony with the nature around me.

Thanks again,
Camel