Goldfish With White Patches

[NeonBlueLeon]

Hello folks,
 
A friend of mine has a comet goldfish in a 30 gallon tank.  His goldfish has semi-transparent white patches on his skin, and its fins are clamped.  I brought my testing equipment over to his place and test the water.  Ammonia and nitrites are zero, nitrates are 80 ppm.  His pH is low, about 6.0.  His tank has been established for years.  Also in the tank are three angels (I know, I know, I gave him an earful already) but they seem to be doing fine.  The tank is heated.
 
Last week, I tested the nitrates and they were off the scale (180+ ppm).  Since then, he's been doing 7% water changes (2 gallons) almost everyday.  Each time, he adds a water conditioner and matches the temperature.
 
Also worth noting, he had two other goldfish before, and both have died unexpectedly a few days ago.  He didn't see any white spots on them before they died, but when I was at his place last week I noticed their fins were clamped.  The goldfish that is still alive was not clamped at the time, but is now.  Anyone have any ideas what could be causing this?  Do you think the water changes are happening too quickly, and the fish are suffering from old tank syndrome?
 
Sorry I don't have a picture.  If the problem persists until next time I go to his place, I'll take some.

 

[Blondielovesfish]

Could his pH of dropped? Do you know what his pH is normally? 
It could also due to them being in a heated tank as they are cold water and it is recommended that they experience temperature fluctuations.

 

[NeonBlueLeon]

Thanks for the reply blondie.  I'm not sure what the pH of the tank is normally, but I do know that the pH of the water coming from the tap is 7.8.  I thought a pH of 6 was too low, even for the angels.  Strange that even with five days worth of small water changes, the pH would remain that low, unless the pH was even lower prior to the water changes.
 
It's strange that these symptoms would have developed now, as the goldfish have been living in that tank for about a year now.  Plus, I would expect the less hardy fish (the angels) to show symptoms of bad water quality before the rock-hardy comets!

 

[Blondielovesfish]

Goldfish are extremely hardy, its how people can keep them in those tiny bowls. I'm not sure why the angels aren't showing symptoms, is it possible that they're showing different symptoms that aren't as noticeable?
Is there anything in his tank that could be effecting the pH? 
Maybe he would be better changing more water. 10%+ would be good.