Looking to buy or make a sampler to capture samples of water below the surface, say down to eight feet deep or more. I bought a dissolved oxygen chemical based measuring kit; it seems to work quite well and is cheap in comparison to direct read systems. Also playing around with a small nano bubble diffuser in a small koi pond. In the big pond (1/10 acre lol), regular aerator diffusers are at six feet depth but the pond's deepest is eight feet. I understand that diffusers don't pump water from the deeper areas (?); I suppose because the deeper water may be cooler and more dense. I haven't tried sampling at depth yet. I see samplers on Amazon and industrial supply sites that cost hundreds, but the dipper design for sampling is not obvious.
DIY: Perhaps a 1/2 inch stick of pvc, stopper one end to hold air, un-stopper at depth, fill with water from the deep, re-stopper and pull up. Or a tube section with flappers at each end, both open going down, then closing to seal the sample when pulling up.
Any suggestions, designs, recommendations will be appreciated!

Dan McWhirter