I think a 2 foot would be a 20. My 20 long is 30 inches long.
pH isn't really going to make a difference in a case like this unless you know for sure that a water change drastically altered it.
You ARE overstocked though, so the new fish could have triggered a glitch in the levels. Of course gouramis can be territorial and aggressive and probably stressed the angels. Also, some gourami species are prone to parasitic disease, but I would think that that would have taken more time to establish in a tank and effect the other fish.
Painted angels you say, as in dipped or injected dyed? If that's the case then these fish were probably already horribly stressed from the process and not destined for a long life any way. The added stress may have been the nail in their coffins.
Incidentally, you do know that that shark is going to way too big for that tank size, right?
