I'm converting my open water Helix 5 G2 Chirp DI unit to use on the hard water. Ive been getting conflicting information on which transducer to use. Ive read to use the 1521 transducer to be able to use the Chirp, but i cannot find it available anywhere. So do I have to use the 1521 or will the XI 9 20 transducer work just fine? Also looking for recommended settings for anyone who has the same 5 or 7 unit. Thanks in advance!