Goldfish deaths

I need help!

My two goldfish are almost dead/dead now I have absoloutly no idea why. Here are the tank specs:
29 gallon Glass, housing 4 2-3" Doublefin Fancy goldfish
1 inch of flourite Gravel
Some live plants for cover, as well as a rock.
A gaint aquaclear filter up to 30 gallons w/ sponge and carbon
I feed them on a regular basis of goldfish food.
All levels are great.

To give you a background, these goldfish have been fine for almost 3 years in the tank, and I haven't done anything different. My dad, without asking last night, did a small water change of about a gallon, cleaned some of the algae off the glass, and added tapwater back into the tank. He also says he added 8 drops of some water de chlorinizer (sp?). The other goldfish are acting as if nothing happened? I did notice that one had scales missing and part of his tail missing, and the other one had a small bump on his tail. They haven't had any of this before! This would have happened within 30 minutes.

Any ideas on the death? These have been here for 3 years, and the other two are doing fine! (The two goldfish that are almost dead have been great the past years, and yesterday afternoon nothing was going on?

Thanks,
PH

I tested for pH, temp, ammonia, nitrite, and nitrate. All were fine?

The water changes were just like anything else. My dad says he added room temp water from the tap, and added the regular amount of de chorinizer.

They actually died later on My favorite ones too..

PH =(