You could try watering down tank water for the nitrite test, maybe 1/4 tank water and 3/4 tap (maybe measure using the Prime dropper or something) and take another reading. At some point you'd expect to get a reading that gives a sensible number on the scale and then you'd know the actual number.
The "feed what your fish'll eat in 2 minutes" nonsense is especially irrelevant with rays. My most voracious rays finish in a matter of seconds and I already wonder if they're over-fed. I wouldn't increase the feeding until the levels are fixed.
Well, I did that... Still super purple. Ugh. I don't know what to do. I won't feed em anymore. I have a bigger problem on my hands lol.
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