Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Test the water. Ammonia? That would kill the fish. If one or two fish had a problem with the water change, and died, that might trigger something like this.
If the new water had something different than the fish were used to, that would stress them.
Higher or lower mineral levels than the tank (GH or KH more than a few degrees off), higher levels of chlorine or chloramine, if you only dosed Safe for a low level.
As for what to do now, a water change won't hurt anything, and would clear the tank water so the light can reach the plants better. Do vacuum the best you can among the plants in case there are some dead fish stuck in there.