help with schnauzer diet

Roxie (3.5) is suffering from urine crystals. She was on antibiotics and Hills CD from the vet for a month. The crystals have eased up but vet suggests keeping her on this diet for life.
The ingredients listed DO NOT look very nutritional and I hesitate to switch permanently. She has been on a Natural Balance food of sweet potato and salmon - no grains. My previous schnauzer (male) lived to be 14 but he had skin problems so had to be on white fish and potato which helped.
I am so confused as to what to feed Roxie.