Never seen 18" hoop nets, you sure it isnt the round traps? If it is hoops, you are gonna have to put them on bottom, so how deep depends on where you are putting them. I use them on a bayou alot and when its normal height I look for some deeper clear holes or around natural creeks, etc... that run into it. When its high, it will get outside the banks and the will run on the bank eating the bugs and all that are forced out of the ground. Thats where I have my best luck.
As far as bait, we have a local trapper supply store and he carries the loaf of cottonmill seed/cheese and whatever else is in it pressed together. Take a hatchet and knock a couple inches off and stick it in the bait bag... your good to go. And always point open end down stream if any current. I assume it is if its hoop nets and not traps.
This post was edited on 1/17 at 8:34 am