My old heater is a 30 gal 240 v. the new one is a 20 gal 120 v. What are the needed electrical changes? Thanks, Jack

With the information provided we cannot tell. At a minimum, the 2 ple breaker would have to be changed to single pole.

The difference in power is not specified, nor is the current wire gage known. A 120 V similar heater would draw twice as much current and the wire size may have to be upgraded.

So we need to know:
Power or current/voltage of both heaters.
Distance from breaker panel.
Current wire size used.

JACKBOY - I can tell you one thing and that's , go back to the 30 gal. tank. The 220 will be less to operate then the 120 volt, and will heat quicker . Now if you decide to keep the 20 gal. 110 volt this is what you will have to do . You will have to go to a 30 amp single
Breaker , and so that someone in the future don't get into a problem ,chanmge the wire to 12/2 with ground, (NOTE IF YOU KEEP THE 10/2 with ground someone may think it is 220 volt, ;;What you save in cost between the 20 gal and a 30 gal 220 will not in the long run save you anything, My advise is to go back to the same you had ; Good luck and stay safe & GOD BLESS F.B.E.