Any other reccent changes? (alk adjustment after getting low maybe)
How do you test your SG? I once saw a tank with a lot of corals looking kind of like your pics and it turned out that the problem was a hydrometer that had drifted pretty far off leading to an elevated SG that seemed to be testing as normal.
Also I have heard of some cases where distilled water can be contaminated with copper if it were distilled using copper coils. Not common, but a remote possibility. Has the polyfilter turned any particular color?