Make sure you're using fresh water, preferrably RO
, to top off your tank regularly. Do this as often as possible to replenish the level back to where it started, replacing water lost by evaporation. As long as you do that, there shouldn't be a big difference in salinity between the tank and the new water you're adding.
If you ever mix your own salt, you need to be more careful about that, but I think the Gulf is a big enough body of water that its salinity should change only slowly and seasonally, if at all.