You should be doing large waterchanges on your rays every couple days. At least 2 a week. It's just standard procedure with rays. If they won't be getting food for a few days, at least they will have clean water
Dude, where did you get this stardard from?. I'm a once a week water changer and my nitrates are 20 ppm not that I I care that they are that low. How do you know his tank size?