Ideally you want to be in the 8.2 to 8.4 range. If the 8.0 is consistent and stays there, you should be ok, but it is borderline low. Any disruption in the chemistry could cause it to drop.
Saltwater in a closed system will naturally drop in PH. That is why buffers are needed to hold it in place. Sometimes it takes additives to boost the alkalinity and buffering capacity. That is one of the functions of the sand and rock - natural buffering. Partial water changes also help by reducing nutrients that draw the PH down.
Don't get comfortable because it's 8.0. Try to figure out why and try to get it up to 8.3 or so.