Usually, a big 'shock' occurs to fish in tanks that have languished for a long time since the last water change. Fish adjust to increasing pollution levels over time (However, their resistance bottoms out and they risk nitrite poisoning). Evidence of this buildup is routinely evidenced when introducing new fish that die almost immediately to soon after being let out of their bags. Quickly dieing feeders is also a huge indication that the tank is souring.
If it has been a long time since any water was changed (refilling evaporated water doesn't count since the toxins are still in the system), begin making small regular water changes. Each week or two, increase the amount of the water being changed until reaching around 50% without seeing signs of stress in your fish. Then, of course, keep up a regular schedule of water changes
(remember the gravel washes, too).