Koi Swordtail Not Eating

[PaBucsFan]

I have had this fish for about 5 weeks now and he has recently stopped eating. I appears to be getting thinner and thinner. I am wondering if he has an internal parasite. My water parameters are great...7.4pH 0's for ammonia and nitrite, and less than 20 for nitrate. I am worried he is going to die from not eating.