I have an aquarium, not too big nor too small.

I have 8 goldfish in it (before they die). I change the water 25% weekly, put anti-chlorine, put 'old leaves' (i don't know whats the tree' name) that stabilize the pH, washing the bottom filter every week. Everything going just fine 3 months after i bought them. I don't see any sign of weakness because all of them are rushingly swim towards me when i was going to feed them.

One day, i change half of the water. As always, i use tap water and put anti-chlorine. After few hours, all of the fish are weak and some are nearly dead. They have something like a 'white-coat' covering all of their body. I clean them out (i can remove the 'white-coat' without hurting them because its like jelly). I think maybe they are poisoning. Some of them are nearly dead and turning upside down.

So, i take action quickly by taking out my quarantine-aquarium and put water from 'mineral water filter' (if you don't know, here is the photo http://tinyurl.com/2vrdub ) into it. Then i put all the fish into the aquarium. A few hours later, miracly they gain their strength and swim happily (especially the one that nearly dead; swimming upside down).

Then, i use chlorine test, then test the pH using Nutrafin pH test kit (6.0 - 7.6) to the 'deadly' water. The result is negative, no chlorine/crystal clear and the pH is between 6 and 7. Then tomorrow, i change the water 100% and use other source tap water. I leave it about several hours then i put all the fish in it. They look healthy as i watch them for about 1 hours. Then i leave them. About 5 hours, all are dead.

The problem is, what is wrong with the 'crystal clear' tap water? How i want to test if it had poison? (other than chlorine test and pH test). Did i missed something? (Forgot to mention that the temperature is room temperature). Thank you.

p.s - Sorry if you did not understand some of the words above. Its because im not very good in english.

You need to check out Robyn's tank pages. I would think it's that white jelly stuff that killed your fish more than the water itself. I'm still learning ponding and no help with aquariums..sorry. ( a ph of 6 seems to be very low for goldfish ).

I'm so sorry about your fish. The white film that you saw on the fish was the fish sloughing off its slime coat. Their skin was literally coming off. That happens in response to something toxic. Their bodies are trying to rid themselves of the chemical or whatever that it is. It's like when you get burnt, and the outer layer of skin comes off. So, what happened? That is not as easy to answer.

Since you did a 50% water change immediately before the problem, something in the new water was most likely at fault. Things such as low oxygen, pH slightly off, hardness, etc. should not cause immediate death. Goldfish like a pH of 7 to 8 but will tolerate down to 6 and up to 9. They won't like it but it also won't kill them, at least not immediately. You should try to get the pH between 7 to 8. A pH between 6 to 7 was not good but shouldn't be deadly. The chemicals that will cause fast death include things such as high ammonia, high nitrite, high chlorine or chloramine (added to tap water), high fluoride (added to tap water, sometimes is overdosed by water companies and is very dangerous), high pesticides or herbicides, and really high or really low pH. I would verify that your chlorine was in fact zero and the pH no lower than 6. Some source water may contain ammonia or nitrite so test for those. Testing for organic chemicals and more obscure inorganic chemicals is much harder and requires a chemistry lab to run screenings on machines like the gas and liquid chromotographs that surround me at work.

How big is your tank? Eight goldfish is a lot unless the tank is huge. How big were the goldfish? What water source did you use the first time the fish almost died and the second time when they did die? It's just your regular tap water? Was it water you had used before? Was the water well aerated? What kind of filter do you have? Most chemicals are clear so clear water can be 100% deadly. I hope you can figure out the problem. Do you drink this same water? If so, you can have the health department test it (include pH, chlorine, fluoride, organics, etc.). There may be something toxic in the water. It needs to be fixed right away. Good luck!