Black Moor gulping at surface

Question
I've been caring for a black moor (named Hammer) after my girlfriend had to leave town about a month or two ago.  He had recently come down with swim bladder disease, but after finding this site and finding a useful cure (and spending a fair amount on water testing supplies and new water conditioner), I'm happy to say he's made a full recovery.  Now I'm noticing that he likes to spend lots of time at the surface of the water, and tends to gulp at the surface.  I used to assume this was him hoping it was feeding time, but after reading up on fish so extensively, I now know this could mean that the tank needs more oxygen.  So what I can do to help the poor guy (or what else may cause this)?

I'm confused mostly because I've been testing the water frequently since got he sick, and have made sure the filter (which is rated for a tank twice the size of this one) is clean, and there is an air pump in the tank to break up the surface tension.  The only problem with the water (as far as the test kit is telling me) is that the pH is high, and I've been treating that with pH down.  I had been making frequent water changes when he was sick, and since then do a 25% water change at least once a week, usually sooner.  So do I simply need to do more to aerate the water, or is there some other cause for his behavior?

Answer
Tim,
 I am hoping that the tank is at least 20 gallons. Chances are the tank is too small. Goldfish are very messy fish. I would turn up the sir stone to try to add more oxygen to the water. Check your filter to make sure it is running properly. A 25% water change every couple of days will help with it too. When the fish do this it is either because there is not enough oxygen in the water, the tank is too small, the filter is not working, or the water is of very bad quality. I would try turning up the air stone first to see if this helps with the problem.