It would be nice to talk to a few others keeping several decent sized rays in low nitrate water that only do once a week waterchanges. Maybe we can pinpoint where the "good" water is and where the "bad" is. Maybe It's the winter? If it doesn't get all cold and snowy then you can get by doing weekly waterchanges?
Then again like said earlier... Maybe it's just me and no one else struggles to keep nitrates down. It may just be my poor filter design...
I'd love to hear from other keepers regarding their WC regimen and nitrate #s.