Seaweed has been used as a fertiliser in many regions where it is easily available.
On some of the British Channel Islands it was a big part of the fertiliser use, possibly with sheep manure. Not sure they had much else on those remote islands.
As you are probably diluting your seaweed harvest for spraying or applying small amounts of powder to soil or pots,
the amount of extra Sodium you actually apply is probably quite low.
You may want to watch out for doubling up the Sodium from other sources, ie your water supply, recycled grey water containing some detergents/soaps, water conditioners.
Otherwise unless you are going crazy, it should be ok.
Any thoughts on the actual Aquarium water, it contains fish wastes.
If you dilute it with fresh, the salt will drop.
I use our freshwater Aquarium water on pot plants when the water is changed.