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?