There was an article at 'the krib' about ferts and water changes. I've long since forgotten the link, sorry. The gist of it was what happens when you add, say nitrate to bring the conc. up to x, then change half the water and repeat the same doseage. The phenomenon under investigation was whether you would end up building up the nutrient to, well, ultimately infinite, right.
Anyway, the person who wrote the article had some talent for math and basically showed that, ignoring uptake (I think) the concentrations of the nutrient would reach a steady state with respect to the % water change and addition. In other words, after a certain number of water changes (the number depending on the % changed) the concentration of the nutrient would be stable.
I think that this will be a similar case, after a few water changes, your Na concs. will level off.
More experienced fish keepers should confirm this, but I've never heard of Na causing a problem with fish. Sec, I wanna check something...