Yes it is OK. A water softener system will not affect the longevity or operation of a Hubbell cement lined water heater tank. A water softener is typically installed when a potable water system has hard water resulting in the need to soften the water. Unfortunately, the salt used in a water softener corrodes the anode rod and exposed steel surfaces of the water heater tank, rapidly causing a glass lined water heater to corrode and leak within a few years. This is an issue for a glass lined water heater due to its reliance on a sacrificial anode, however because a cement lined tank does not use a sacrificial anode softened water is of no concern to the Hubbell cement lined tank.