Dying fish

The water was brown, probably from over feeding, but instead of changing the filter my roommate decided to change the filter and the water at the same time. My roommate remembered me saying never to change out more than 25% of the water at a time, but lost track of the siphoning action while changing out the filter.  Only after changing the filter out did my roommate notice that over 75% of the water was gone. Now (2 hours later) one of my fish is dead, and the others are swimming on their sides (big heads and algae eaters) and probably dying.

This is the second time this has happened. The last time all of my fish died. My question - is there anyway to prevent the deaths when someone makes a mistake like this? What should I do when I find my fish floating around obviously in shock?

30 gallon freshwater tank
seven fish (a year and a half old, each weighing approx. 12 - 16 ounces)
Water Conditioner added

Dying fish in San Diego

[Aruba417]

What kind of fish are they? Did you add the Stress Coat stuff (that might be the same thing as water conditioner...)? Did you test the PH?