hot springs

We just bought a used hot spring spa and there are sinking black flecks (seem like tiny rocks). They don’t disintegrate upon touching. Any insight? Happened before we even got in it but was clean when we loaded the water. They treated/flushed it before delivery. Thanks in advance!

Hello everyone and Happy Holidays. I have been waiting patiently for a deal and one popped up in my area for a used hot tub. I purchased a 2007 Hot Springs Envoy which is a 5 seater 450 gallon tub off of Facebork Marketplace for 600 bucks. Here's a photo of the listing. The guy selling was...