Freshwater fish tank

Question
I have a 30 gallon tank with guaramies, platties, black mollies, cat fish and a sucker fish.  I've had it for a few years now with generally no problems.  However, yesterday, I syphoned the tank, changed the filter, and changed about 1/3 of the tank water.  This morning, I came out to find a couple of fish dead and then several others started to die as well.  I lost about 12 fish altogether.  I immediately syphoned out some of the water and added Easy Balance which seemed to help.  A couple of the fish actually came back to life.  The water is still a little cloudy though.  What could be the cause.  I feel terrible!

Answer
Hi Jeannine,
How long has it been since the water in the tank was changed?
If it was more than the usual time in between then likely there was a pH/ water chemistry shock. This happens when old water in the tank sits so long and acids naturally produced by decaying matter lower the usual level of pH in the tank. However, your tap water, since it is fresh, has a higher pH so when doing a water change, the pH suddenly fluctuates and the fish can really suffer.

Also did you make sure to add water conditioner to the new water? This is very important because there are too many harmful chemicals like chlorine and chloramine that really can harm fish and you must be sure to use water conditioner every time you do a water change.

The best way to prevent pH shock with new water is to simply be sure to maintain a consistent reasonable water change schedule on the tank which helps prevent buildup of organics that lower the pH.

If the aquarium has gone a long time without a water change likely there is a great amount of organic buildup which is best handled with more frequent partial water changes. Try to do 30% or 1/3 twice a week.

Water changes are the best thing you can do for your fish... And maybe the real reason was something was seriously off with the tap water that you were not aware of. We can never be for sure.

But I do hope these tips help in solving this mystery.
Good luck! Hope the rest of the fish pull through!
Susan~