Alternatives for prescription diet on UTI with crystals

My almost 4 year old sweet tuxedo (Tucker) cat was diagnosed with UTI crystals on 11/2014. After putting him on PUrina UR St Ox cans for only a couple months with other dry foods his UTI crystals came back again on 8/2015. I was not happy with the previous vet's diet treatment on his UTI with crystal condition, so I took him to another vet after finding out his crystals came back. The 2nd vet suggested that Tucker needed to be on Hill's SD for a month to dissolve crystals, then on Hill's CD or any other UR prescription diet FOREVER for maintenance. Hill's SD did work very well and dissolved all of his crystals. 

Now I put him on prescription C/D the chicken flavor but he is tired of eating the same food. I switched to tuna/vegetable flavor and he likes it. Well, I've heard that not to give fish diet to your cats on a daily basis. Does anyone know if Purina Pro Plan Urinary Track Health canned foods or any other wet foods will do the same as C/D for long term maintenance and to prevent forming crystals? 

My cat had a blockage about a month ago and wouldn't eat the Hill's dry or canned so he is now on Royal Canin SO canned. Even with that I am having to mix a little Fancy Feast in it and usually sprinkle some  freeze dried salmon treat on it called 'PureBites'. I have asked on here and several have said that little bit of fish on his food won't hurt. This food gets expensive but I don't want to see him be uncomfortable and have another expensive vet bill. I still owe several hundred dollars for the treatment for his blockage.

Royal Canin food from what I understand has a very high success rate at preventing reoccurring problems. Here is a thread I found

http://www.thecatsite.com/t/246798/royal-canin-vet-diet-urinary-so

Answer to Molly92- yes my cat was on dry food 95% of the time when he developed the crystals. The dry foods he ate were Orijen, Fromm and Pro Plan Weight Management. Pro Plan Weight Managemnt was recommended by my vet and he told me it would prevent him from developing crystals, and he was totally wrong!!! That's why Tucker my cat developed crystals the 2nd time and I had to switch him to another vet who was able to treat him with correct prescription diet.

I do notice that Tucker doesn't like to drink water, so right now I have to squirk water with syringe into his mouth. Luckily he is an easy cat to let you squirk liquid into his mouth. My 2nd vet told me since he has the history of developing crystals he needs to be on prescription diet forever for maintenance. Based on the link form Catlover100 sent, and other online research, I think it's necessary to at least keep him on prescription diet 80% of his meal. I'll put some other wet food topping in his prescription diet food to encourage him to eat.

I might let him try Royal Canin SO. It has lots of good reviews. I'd rather spend $$ on food than on the vet visit for another crystals. I know these ingredients are terrible, but guess it's better than blockage. Tucker likes Purina Urinary Tract health canned food.... I might mix some of it on his prescription food.