Help! I always had great hot water pressure. I also had a leak in my bathtub tap, that leaked between the walls. Tore out the wall, and found the leak, turned out I just needed to turn, one complete turn of the bathtub spout. The leak completely stopped. Problem now is little to no hot water pressure. I also had a new hot water tank installed 4 months ago because the old one had sprung a leak.
Any help would be appreciated!

Somewhere along the line of jobs done you have likley knocked loose some junk in the lines. Is the low pressure/volume just on the tub or all fixtures? If just the tub pull the cartridge, clean it, flush the line and reinstall