Problem reading deep water 400+

I have a had problems reading deep water with my electronics systems. I had a DSM 300 and now have a new CP370 and I am unable to see deeper than 400 feet. It reads strong bottom up to that depth then the screen scatters as seen in the attached photo. The transducer on the boat is an Airmar M 260 (1kW) shooting thru hull. The transducer tank is topped off with mineral oil.

How can I get to be able to read depths to 1000 feet? The transducer tank has run low on oil could the transducer be burnt?

thanks

Chris D.

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