John, your Fishmark 320 does have that capability (1500 watts/320 vp) which is why I listed it as a minimum base unit. I have the same model on my boat along with a more powerful X-15MT. Just check to make sure you're not running any high filtering on the unit that would filter out those small returns. I would also check to see if you're running in auto, what sensitivity it typically is running at. You might need to override that part in manual (bump up to 85% or more if screen clutter allows). Other than that you should be good to go.

As a quick example, I was out 2 weeks ago and was able to watch my 1/16-oz. crappie tube on my 320's screen in 25' of water. So I know it can be done (LOL).

-SBB