I wouldnt do that large of a water change. Just do smaller ones (like regular size) more often for a while (like if you do a 20g change every 2 weeks, do a 20g change once every week). I'd do that for a couple/few water changes and then go back to normall. But imo, you really need to do all your tests. amonia, nitrite, nitrates, pH, sg all that. And if you got corals, you REALLY need to find out what all the other stuff like calcium and mag and all those are.