mmm, we all did things in "the old days" like that BUT (sorry Caryl), you should not even try that in a marine tank.
I hope this tank is mature (at least 8 weeks for just part of the cycle) before anything inclding weed has been added, and like someone else said, you are doing daily water changes, if no filtration.
How long has your tank been set up?? If recently, those seaweeds are going to die, then pollute your tank.
Could be wrong but i think you need to do some more research.
You certainly do not, repeat do not, top up a marine tank with seawater, water changes yes, topping up no - salt does not evaporate and all you will do is slowly increase your salinity until everything dies.