what are these hard white deposits?

[Lode]

It's common to find teeth of the rays in a bare bottom tank. Especially with larger rays. It's not clear on your photo, but I wouldn't be surprised if this is what it is in this case also

 

[Nic]

Ebay microscope probably < $50 for a reasonable one,
buy one and find out for sure for yourself wtf you got there.
Everything else is just a guessing game

cheapest on ebay i saw for a decent one was 225$

 

[DavidW]

<$10
http://cgi.ebay.com/New-60X-100X-Li...trkparms=65:12|66:2|39:1|72:1240|293:1|294:50

 

[ShadowStryder]

Microscope isn't gonna do someone like me any good when I don't have a clue what
I am looking at. Unless of course it is moving, then all it tells me is that it is alive.