My fish keep dieing right after the other $70 of fish down the drain?

[Theresa P]

Hi, i have an fish tank its 30 gallon size with a big filter on the back. I had a perfect aquarium at one time everything had been alive for about 8 months. But me being stupid found a piece of coral reef out of the ocean and thought it would be neat to add to my tank and killed everything I guess from the salt. But since then I have took the reef out cleaned the tank with a water hose boiled all of my gravel and decorations. Filled the tank back up added some start up chemical to get rid of any chlorine and so forth. Put in a number of goldfish and tropical fish and they all died except 2. I figured the salt was still in the tank some even though i also changed the filter cartridge so i didnt think nothing of it. By the way the 2 that lived was a silver goldfish and a red fin shark. So after they survived i kept them in the tank alone for about 3 months recently i added some more tropical fish and a betta and everything is starting to die off again 8 deaths in 2 days. My water is a little cloudy and my silver gold fish that lived through everything is now orange with just a hair of black i thought that was odd. Would someone please help me. My children enjoy watching fish and i would sure enjoy not having to keep buying expensive fish that kick over dead in about 5 days. Please help