Hi guys. I'm new here, but your information has been excellent. I'm trying to improve our Murray river turtle's tank. The tank came with a sand, which we took out. How can we tell if it's river sand or beach sand? It has a few little shells in it, but that could be shell grit too. If it's river sand, I will put it back in with calgrit. Any ideas? Photo of some attached. Thank you! Regards, Wendy.