Use the "Upper Range" setting to eliminate "x" number of feet from the upper part of the displayed imaged and you won't have the plankton cluttering up your screen ...

Plus the remaining screen gets used by stretching out visible structure vertically on the display ... Making it "sort of" a zoomed image ...

Of course ... If the plankton covers the entire water column ... Only a lower Sensitivity setting (or other filter setting) will help in his case ...(which also makes good target echoes less prominent on the display) ...

Rickie