Update: I haven't replaced the sand or washed it with tannic tea, etc...but when I came home yesterday, it seemed rain had already done the job for me. It was flooded to the brim and seemed to have helped the overall health of the bog tray system. I see lotsa sundew seedlings floating and the moss seems to be turning lighter green, as opposed to the dark algae-covered mush. I think the rain probably washed out the salts or at least diluted it--it might turn out good afterall!
Thanks again for all the help.