Time for a trip to the vet for a urinalysis to determine what type of crystals are present. (struvite? calcium oxalate? other?) Best to have the vet collect a fresh urine sample in the office rather than bringing a sample in as some types of crystals can form in "old" urine.
Sometimes a change in diet (to adjust the urine pH) helps. Your vet can advise you on treatment based on the type of crystals present.
Vet for a U/A. Even if you feed wet food, if it's got a lot of phosphorous and magnesium in it (think Alley Cat, Meow Mix, etc) then you can have trouble even with a normal kitty. Some kitties have more serious problems though even on good food and need meds to help with it.
A good horseman doesn't have to tell anyone...the horse already knows.
most (not all) cat crystals are, ultimately, the result of dehydration- cats just don't drink well, they don't seem to have a strong "thirst" instinct. They are designed to eat a very wet diet of raw meat which should provide all of their water; when we feed them dry foods, they often fail to go drink. Even commercial canned foods are "drier" than the natural prey cats are designed to eat.
So you say the cat eats "mostly" wet food- well, I'd switch that to "all" wet food. You might want to boost the water content of the wet food by adding some chicken broth or clam juice to it. You want the cat to consume a lot of water, and to pee often, which you should actually notice as a need to clean out the litterbox more often. This will flush out the cat's bladder, prevent bladder stones, and may help prevent kidney disease later in life.
I know some people who don't offer regular water to cats- they put out bowls of tasty chicken broth or clam juice to encourage cats to drink more, more, more.
I'm kind of like a broken record, but it really has some great info on cat nutrition, wellness, and urinary tract issues. Really opened my eyes when my male kitty almost had a blockage. After following her advice, we've had no further flare ups. Worth a look.
The basics: wet food diet and grain free (even if your kitty protests mightily). Lots of encouragement to increase water intake.
I have a cat that had crystals. He was on a prescription diet (cringe). I switched him earlier this year to TCFeline. A premixed raw diet that I add 2 lbs of turkey or chicken. He is doing great. It's easy to make and feed. I can mix and portion a batch out in about 15 min. One batch makes about 20-22 cups. I freeze most and thaw out a couple of days at a time. Pop the lid. Drop the cup in his feed bowl.
\"You have two choices when a defining moment comes along - you can either define the moment, or let the moment define you.\" Tin Cup
I don't know if this is even relevant for cats, but my dog consistently had crystals in her urine. Vet kept telling me to increase her water intake, put water in her food, etc, which didn't help at all. Finally one of the vet techs mentioned that our area (near the foothills) is known for rocky soil and therefore a high mineral content in the water. I gave her distilled water only for about a month to be sure her system was flushed out, then installed a Pur filter on my faucet, and she gets only filtered water in her bowl. Vet hasn't seen any crystals in her urine in 2 years now, and she seems to be much less prone to UTIs as well, although that could just be coincidence. I don't know if that even applies to cats, but it's a very easy thing to do, so might be worth a shot. I've since heard from a lot of people in my area who have to do filtered water to avoid kidney stones, so at least she's not the only high maintenance one :-)