clowdy water and dying fish

Question
Our tank has been set up for about three weeks, it is a 37 gal freshwater tank with non-aggressive fish.  We started out with 12 fish and now we are down to 4.  We have a cascading EMPEROR 280 Bio Filter.  I have not yet changed the water...
Shortly after adding the fish to the tank, we noticed that the water was getting cloudy, then the fish started dying off slowly.  One of our fish is more aggressive than the others and picks at them, shortly after, the ones he was picking at ended up dying.  I considered that the foggy water was because of the dying fish but now I am not so sure.  I went to the aqua shop and they gave me a refrigerated solution to kill off any bacteria, but that has only made things worse.  I am taking a sample of my water to the aqua shop today to have them check it out, but so far they have not been too helpful.  Any suggestions???  We are new to this.

Answer
Hi Erin;

It sounds like a classic case of "New Tank Syndrome" with way too many fish to get it through the break-in safely. Make a 25% water change right away and do another 25% tomorrow. Toxins are killing the fish because the beneficial bacteria that is supposed to handle them isn't mature enough. Cloudy is water is a common symptom. The refrigerated solution probably was not a product to kill bacteria, but to supplement the beneficial bacteria that is struggling to develop. It may seem worse because at the 3 week period there are two toxins that are usually elevated at one time, ammonia and nitrite. Read through the following web pages to learn more about what's going on in there;

My own web page about new tanks;

http://www.xanga.com/Expert_Fish_Help

A few other helpful pages;

http://www.firsttankguide.net/

http://fins.actwin.com/mirror/begin.html

http://freshaquarium.about.com/od/aquariumstartup/a/newtankmistakes.htm

http://freshaquarium.about.com/cs/biologicalcycle/a/nitrogencycle.htm

http://freshaquarium.about.com/cs/waterconditions/a/cloudywater.htm

http://badmanstropicalfish.com/fish_chart.html

http://freshaquarium.about.com/cs/beginnerinfo/a/fishcalc.htm

It would be helpful to know what kinds of fish you have. If one is being aggressive it may get worse as tank conditions improve and he feels more energetic. If you aren't sure, try to get a photo of the fish and send it to;

AllExperts_Fish_Guru(a)yahoo.com

Followups VERY welcome...

At Your Service;
Chris Robbins