I can't be sure cuz i have gravel not sand (smooth round rocks, they pound down on it and send them flying everywhere, they love it), but i doubt any of my rays would bury if i had sand. I mean these rays can bury themselves in the gravel, my retics used to do it, they just don't. I've seen one of the females bury once, after I disturbed my filter bed and probably had a bit of a nitrate spike, but it was for like 5 minutes. Even introducing them they never buried right away. Maybe they feel comfortable just feeling that they would be able to, but i feel like they don't feel stressed bad enough to bury once they're older really. My 0.2. I also never had any tank mates that would remotely be a threat to them either.
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