All fish died over night

Question
QUESTION: I have an established 10 gallon tank, that all of a suuden every fish died over night.  I did a 10% water change the day before, and water was clear and fish were doing well.  Next morning all fish were floating and water was cloudy.  Tank as been up and running for about six months and has liv plants in it.
Thanks

ANSWER: Hi Alan,
When more than 1 fish die, 9 times out of 10 its the water quality.  And when you state all fish ended up dieing then I would believe water is the culprit.  How often do you do water changes?  in 6 months with rarely any water changes done will result in high nitrogen levels.  Water changes must be done weekly to preven nitrogen toxin build up.  Did you condition the water before putting it in?  Did you accidently "condition" the water with some other liquid?  Since you have live plants, are you using CO2 and did you overdose?  Or overdose on liquid carbon?  Whatever the reason is, try to remember what could have possibly happened because I'm 100% sure it is the water.  Something must have gotten in it or something happened to the water.  
-Matt-  

---------- FOLLOW-UP ----------

QUESTION: I do 25% water change once a month, and 10% the remaining weeks of the  month. Now when I did the last 10% change, which was yesterday, I put in condition amount for what is recommended for 10 Gallons. Can two much conditioner kill the fish?  I not using Co2 and not sure how thaqt is used. The water is yellow right now.  The container I use to refill tank is not used for anything else, and I have been using the entire 6 months.  What should I do now? drain all the water and get new plants? I haven't been conditioning tge water before it goes into the tank. I've been just putting the conditioner in after tank is filled.

ANSWER: If you are not using any rich substrate and are only using plain gravel with a couple of live plants, I would drain 100% of the water and add new water.  Do not wash the tank down with tap water because it will kill the beneficial bacteria that is necessary to break down ammonia and nitrite caused by excess food and fish waste.  Condition the water before adding it to the tank.  And too much condition will not harm the fish.  Do partial water changes weekly rather than monthly.  Monthly results in too much toxin build up and will harm the fish's gills, and may result in death.
-Matt-  

---------- FOLLOW-UP ----------

QUESTION: Thanks for your input.  Still confused and trying to figure out what comtaminated the water.  Will outdated or bad frozen food like brine shrimp kill all the fish over night. What the fish store told me to do since it was a small tank and not alot of fish, was to put about a weeks worth thawed in frig in a container, because it comes in large chunks too much for the amount of fish.  I forgot to say in previous messages, that I did bring in water sample and everthing according to them was good.
thanks again.

Answer
It will only kill the fish if the food has been left in there long.  The organic materials will decompose into ammonia and will cause ammonia intoxication.  Pet stores when they test, usually do test strips.  It is very inaccurate.  In order to accurately test the water parameters, you need to buy liquid testing.  Such as the API brand that tests for ammonia nitrite and nitrate.
Anything above 0.50ppm Ammonia, 0.50ppm Nitrite, 40ppm Nitrate will harm fish.
-Matt-