Who hears the fishes when they cry?

I am sad to report that I had a fish tank crisis over the weekend. I had 5 medium sized goldfish in a 10 gallon tank. I knew the water was getting close to needing to be changed when I took a close look on Saturday morning. However, by Saturday night when we returned home, I had a crisis. Due to the size of the fish and an aging (or possibly faulty) filter pad, and the nitrates in the water (from fish and food waste) the pH dropped below 6.00 and caused the fish to actually be suffocating in their water. One fish was found dead. The smallest one.

I worked hard draining the tank, and adding various things to bring the pH up to 6.6 that night. I added some medicine to help the fish. However, by Sunday morning a second fish was gone. The fancy one. I did a water change on Sunday and just checked the pH now - and it's at 6.8 - which is almost neutral.

They are still showing mild symptoms and haven't really been eating, but I'm hopeful more won't be lost. These three fish should be the right size for the tank as long as they don't get bigger (I had to sell one about a year ago who was over 8" long! ~ too big for a 10 gallon tank.) I am monitoring the tank very closely and sure hope I don't lose any others. I hate when pets die.

"Away with the superficial and selfish philanthropy of men; who knows what admirable virtue of fishes
may be below low-water-mark, bearing up against a hard destiny, not admired by that fellow-creature
who alone can appreciate it! Who hears the fishes when they cry?."
---Henry David Thoreau, from A Week on the Concord and Merrimack Rivers