unclenedsfishfactory.com

So I have had a new tank for a couple of months and everything seemed to be going well until one of my Corys died recently. I didn't think that anything was really wrong, but I did find it strange that the fish were losing their appetite and hiding a lot recently. Now that there was a death, I know that something is wrong so I have been inspecting them more and it seems that my Corys have some fin damage which I am assuming is a case of fin rot (yet my Angels and swords are fine).

Any idea what could be causing it? I have been checking the pH, ammonia, and nitrites and those are 7.4, 0, and 0 respectively. The temperature is between 76 and 78 degrees across the tank, which I think is ok, but other than that I don't really know what I could be doing wrong. Any thoughts?

I would change the water
(maybe 50% every other day for a week, use water conditioner for the new water)
and do a gravel vacuum at the same time...
without doing any harm, you can try a mild antibiotic like Erythromycin
(remove carbon from filter, follow package directions)
...did you use a weird gravel...like ground glass from Walmart or some Chinese made junk?