Replacement O-Ring for SRO CR5000 too small?

So, I have a reef octopus calcium reactor (I think CR-5000 https://www.coralvue.com/reef-octopus-sro-cr5000-calcium-reactor)and I was going to grease up the o-rings on it. I took the lid off, and when I tried to put it back on the O-ring was larger than it used to be apparently, by about 1/2 inch. I was so perplexed, I tried everything to get it back on, but no success. I even tried boiling it so it would shrink back.

Ultimately, I went to CoravlVue and tried to by another O-ring.
https://www.coralvue.com/sro-calcium-reactor-lid-oring
But when it came in today, the O-Ring was too small; and I had to stretch it for it to even fit in the groove. And of course, since I was stretching it; it just kept popping off as soon as I took my hand off of it.

I honestly have no clue what to do, I'm not 100% sure the model of this calcium reactor, but I do know that it has a 9inch body, and the diameter of the o-ring groove is about 6 inches.
I don't believe I have the CR7000, but I have no clue what it looks like or the specs, because google is not turning up anything.

Any help would be so much appreciated. Thank you
 
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