He had a urinary obstruction in past. I add about 50 ml to his wet urinary prescription food. I'm wondering if I'm

I'm glad to hear Yao is not having any symptoms of urinary obstruction! It is a very good idea to add additional water to the canned food of these cats.

The average 10 pound cat should drink at least 8 ounces of water per day, although most cats don't manage to drink this amount on their own. Cats that are not good water drinkers can be more prone to urinary issues, including obstruction, kidney problems, etc.

50 ml is just under 2 ounces, so I don't think you are overdoing the supplemental water unless Yao is a very good water drinker on his own. You could try measuring out his water once daily and then measure it again 24 hours later to determine how much he is drinking on his own. Do this on several different days so you can determine the average amount Yao drinks per day as he may drink more or less than usual on any specific day.

Depending on Yao's body weight, if he is drinking at least 8 ounces per day on average on his own per 10 pounds of body weight, then adding 2 additional ounces of water to his food could possibly be overdoing it, but there is a good chance that Yao does not drink that much on his own.

If Yao's total daily water intake Including the additional water added to his food is significantly more than 8 ounces per 10 pounds per day, then I would decrease the amount of water you are adding to his food. Otherwise if Yao's total daily water intake is about 8 ounces or less per day, then you may want to have your vet check some bloodwork on Yao to make sure he hasn't developed kidney or liver issues, diabetes, etc, which can also cause cats to urinate large amounts.

My guess would be that Yao is receiving about 2 ounces more water than he has ever drank on his own, which is causing him to create more urine than he has previously. In most cases this is a good thing with a cat that has previously had urinary obstruction as it helps to keep the urinary system flushed out and healthier. I hope this helps and that Yao does well for you!

Thank you for the additional information. How old is Yao?

Thank you. What is his urine pH, and what brand of food is he on?

Ideal urine pH for a cat of his age would be 6.8 - 7.2  A good food for him would be c/d  Multicare Stress Urinary Care, which helps with preventing both struvite and calcium oxalate stones, and also helps relieve stress.

It has ingredients that help relieve stress like L-Tryptophan and hydrolyzed casein

If it seems like he was less anxious and stressed while he was on it, he can take Gabapentin long term

What is Yao's urine pH?

You're right; he could be more prone to developing calcium oxalate stones, but other than that he should be fine. It is not unusual for a cat to have acidic urine, but is not preferable as they get older

You're very welcome, and I hope Yao does well for you!