My goldfish kept dying

Question
I had been keeping fishes for a few years.I have two 3ft tanks with full filtration systems.One day, i make a 3/4 water change in both my fish tanks.About an hour later, my goldfish die follow by my dragon fish.After their death, I clean up the tanks throughly and changed new filter wool,add in new water and let the filter run for a week before adding new goldfish.Within a night, all the goldfish die.Before they die i notice that they are lying listless at the bottom with red tail and fins. this time, i again  clean up the tanks throughly and changed new filter wool,add in new water and let the filter run for a week before adding new goldfish.This time, i put 3 goldfish in each tank.An hour later,Fishes in tank A started hanging and soon their fins and tails are stain red.I quickly took a clean pail(A) and add in freshwater and a little salt than put the fishes in with only air pump.The fishes got better after about 3 hours.In tank B,fishes suddenly lie listless at the bottom at a corner but does not have red tail and fin.Again, I took a clean pail(B) and add in freshwater and a little salt than put the fishes in with only air pump.The next day,fishes in pail A are doing fine but two of the fish in pail B are dead.Now I have put all the survived fish in pail A.They are doing well and red tail and fin had already disappeared and no longer hanging.Two of the fishes had already started eating.I would really like to know why is it that although the tanks are clean thoroughly but each time I put new fishes they still die.Is there anything wrong with my tank? How can i solve my problems? Thanks

Answer
Dear Serene,
What seems to be happening is the goldfish are having a severe reaction to very poor water quality. Ammonia is a very dangerous compound that affects goldfish very severely, they can develop redenned fins, lay on the bottom and the mortality rate is very high. Ammonia can come from an overcleaned tank in where the beneficial bacteria who convert ammonia into safe compounds are unknowingly destroyed if the gravel is cleaned too throughly or the filter wool is changed completely. Unfortunately this could certainly be your problem here. Without any beneficial bacterial colonies, and the fact that goldfish are very high producers of ammonia this toxin can rise at an alarming rate.

Also some things to consider are- were you sure to use dechlorinator or water conditioner to remove chlorine and chloramine when doing the water changes? There could also be heavy metals in your tap water but fortunately most water conditioners will remove this as well. If you didn't use water conditioner your main source water may be the problem here.
Also something to consider is did you ever use any kind of chemical when cleaning your aquariums? And is it possible any kind of chemical got into your tank, whether it be aerisol sprays, incense, and other such things.

These are all very important things to consider as to why your poor goldfish are suffering so terribly bad. A very good thing to do is to try testing your aquarium water with an aquarium test kit to check your ammonia, nitrite, and nitrate levels.
In healthy aquariums--Ammonia and nitrite should be ZERO and nitrate less than 20 on the test kit readings.

What I suspect highly is your aquariums have some very dangerous water quality problem affecting the goldfish. It may be best to do some thorough investigating by testing your water, checking if it has any odd smell, making sure no chemicals have been used around the aquarium or in it.

If you have anymore questions, feel free to write! I am more than happy to help you solve this mystery.

Best wishes and Happy holidays!
Karen~