some salt maybe? put it in the sump or disolve first if you dont have one.
To those commenting about the light difuser, I used pipe insulation on the edges so I can kind of wedge it on the acrylic without scratching, but it's become irritating because it floats and is a bit easy for a ray to knock over, what method did people use to cut it to size? Acrylic scratches extremely easy so I chose to be safe rather than sorry, plus I did a crap cutting job, so was wondering how people cut it to size.