Best food for cats with cystitis

Hi ...
One of our cats has bouts of cystitis and a person at work said she was
told, don't know by whom, that cats should not eat canned foods with fish in
it if they have cystitis.

Is anyone have knowledge of this and if so what would be the reason? The
only canned food Saffie will eat is Fancy Feast Fish and Shrimp.

We want her to eat more canned food because all dry food diet isn't good for
cats with cystitis or crystals.

Thanks, Rick & Rob