A proper cat diet i.e. high meat (no grains/cereals/veggies) wet food is certainly more suitable - and palatable! However, diet isnt necessarily a 'cause' of flutd, it can be a combination of things - diet may be partly responsible, but being overweight and/or inactive can also contribute, as can low water intake - and stress is a often a major factor. So diet alone isnt always the answer, you might want to look at things like Zylkene if stress is a factor, and perhaps Cystaid which is said to help sooth the lining of the bladder. Fish is actually quite high in magnesium and could contribute to crystals, if a cat is prone, I understand.
My cat Ben had a blockage over 5 yrs ago now, (think it was partly diet - at the time I only fed dry, and partly stress - had moved house) and I changed his diet to raw meat plus high meat content pouch/tins (applaws,, hi-life etc) and he never had another problem.
