1. I had a similar situation with my skimmer. What I found was one large grain of my coarse sand subtrate had found its way into the intake of the skimmer and had lodged itself in the intake where it tapers down, just before the opening for the air intake. This cut the flow of the water at this point and limmitted the ammount of air intake. the air hole wasn't blocked at all, so when I blew into it, it seemed to work fine. I had to take the whole thing apart to find it, but when I put it back together, it worked great.
2. Nitrates bad!!! You need to keep up with water changes but I doubt that nitrates are your problem. You noted that you made two major changes recently. You stopped using a certain supplement and you changed your salt. I don't put much faith in supplements so I don't believe that they would be your downfall either. I would think that the change in salt is your culprit. To you and I, Salt is Salt. But to a reef, even minor differences are a big deal. A while back, I switched from Intant Ocean to Ocianic Salt and my tank did not like it at all. I stuck it out and my tank looks good now, but it was a scary couple of weeks when I changed over. The only other thing I could think of would be a temp. spike which isn't totally out of the question with recent weather.
2. Nitrates bad!!! You need to keep up with water changes but I doubt that nitrates are your problem. You noted that you made two major changes recently. You stopped using a certain supplement and you changed your salt. I don't put much faith in supplements so I don't believe that they would be your downfall either. I would think that the change in salt is your culprit. To you and I, Salt is Salt. But to a reef, even minor differences are a big deal. A while back, I switched from Intant Ocean to Ocianic Salt and my tank did not like it at all. I stuck it out and my tank looks good now, but it was a scary couple of weeks when I changed over. The only other thing I could think of would be a temp. spike which isn't totally out of the question with recent weather.