So my nitrates shot up sky high. Could it be a bad test kit, or can I do a 75%wc without hurting my rays? I'm picking up a fx6 cannister this weekend to help keep up the filtration.
As long as the water you add is the same parameters as the tank, you could do 100% water changes. Obviously the water will need to be treated, i usually double dose dechlorinator with a large water change. (Just in case)
As for adding another filter... It will do nothing for your nitrates, your bio media is purely for converting ammonia into nitrate. Nitrate is then removed via water changes.
I always do 65-85% water changes. I do not dose the new water at all. I also don't just fill the tank all the way up. I measured how much water flows from my tap in 1 minute. It is 6 gallons. I fill for 3 minutes, which is 18 gallons. I wait 15-30 mins because I be doing other things and the the fill again for 3 minutes. I do this until they are full.
If your nitrates shot up sky high, then other parameters most likely have also shifted due to the rise of nitrates, such a pH, TDS etc. In this case, with rays, I would proceed with caution and perform a few smaller water changes instead of one big one.
You didn't mention your w/c routine, but going by the high nitrate, it appears it isn't enough. I agree with RD that you shouldn't go with a big water change as of yet. I would do about 30% everyday for a week and then after that start a regular routine of at least 50%, at least once a week......but more is better, as long as you stay consistent. As Lgw posted, as long as parameters and temp are about the same, its possible to do 100%......you don't want to do 100% w/c every 2 weeks