Table salt is NOT ok. Table salt contains yellow prussiate of soda (Sodium Ferrocyanide -- an iron salt) as an anti caking agent and it can and will cause iron stains.
Use solar salt or salt pellets made for water softeners. Much cheaper than pool salt and often purer!
Personally, I use either Morton or Diamond Crystal solar salt (about $5/40 lb bag).
It's much faster dissolving than the pellets. Almost as fast dissolving as the much more expensive pool salt! Then again, does it really matter that much if the salt dissolves in 1 hour or 3 hours? It will still take a full day of circulation to really mix it throughout the water!