I trap minnows all the time out of local creeks. My trap always rests on bottom and I do very well. Last year we went camping at a local lake and I set my trap off the docks. I didn't catch a single minnow. The next day I set it and I watch it for about 30 minutes. All the bread and dog food floated to the top and the trap and the minnows were able to pick at the bait through the holes in the top without ever going in. I wrapped my bait in cheese cloth and hung it with wire so it was directly in the middle. A couple hours later I had a few dozen.

Since you have dock slips, I would recommend an umbrella dip net. Throw in some bread piece and pull it up when minnows swim in, instant gratification