Keeping minnows alive in the lake

I have a box that my dad built many years ago with screen mesh. Last December I bought several dozen minnows and kept them alive for 3-4 months over the winter with the box at the surface tied to a post in our dock since the water temp was very cool (unfortunately the fishing wasn't any good -- that's why I still had minnows for that long). Over the last month or so, as the water temp has gone up, I started losing minnows and getting a lot of algae growth on the box. So I dropped the box much deeper (guessing 15-18' deep). But the box was covered in silt and the minnows were not only dead, but disintegrated. So now I'm trying to determine the perfect depth to keep them cool and keep them alive as the water temps are still rising.

Any suggestions for depth? Is there not enough oxygen at certain depth for the minnows?

The magic depth is going to be the coolest zone (deeper water) with the most oxygen (closer to the surface). So what's that? Not sure if any of us can figure that out unless we have scientific equipment on loan from NOAA. I just place a rock in the minnow basket that will sink it down about 3 feet below the surface and hope for the best. Usually lose minnows but have enough to fish with, but its a hassle in the months when the water climbs above 70 degrees. I mostly fish with jigs in the summer. Good luck Steve and by the way, your ramp is looking good.

What your looking for is the thermocline. The coolest water will be below it, but the most oxygen will be above it. Its like when you dive into the lake, you will pass into a zone of cold water. That's the thermocline. It can vary by the day. The way to find it from a dock is by a remote thermometer. I have one of the old combo color collectors that has a thermometer on a line. I can read it from the surface. Drop it into the water, and when the temp gets a sharp decline, you have found the thermocline. I scuba dive, so when on Beaver lake in Arkansas and tenkiller lake in Ok, I've always seen the fish stacked up about a foot or so above the colder water.

Thanks guys for the information. Glad to know it's not as easy as I was hoping for - I thought I was just making this more difficult. Dennis, I know what you mean about that cold layer when you dive in - we dive in off the dock and hit that cold water down deep. It feels very refreshing when you bring it back to the surface when the surface temps are warm.

Thanks Jim, seems to be constant work maintaining the dock. Should get the rest of the material for the walkway this weekend. I need to cut a few of the angle iron supports that I had installed before - then weld them again to support a few ends of the new stuff.

Dang it Stink you need to increase the size of your text I can't even begin to read it. Even when I enlarge it on my phone I can't get your text on the screen without a lot of scrolling. Be Kind you OLD BIRD to us older folks.