It sounds like the most likely cause for death was too many fish being introduced way too fast to a tank that hasn't been cycled properly (I know you left it running, but as others have said, without a food/ammonia source, your filter bacteria would not have grown in your filter). 8 fish is a HUGE amount of fish to add in one go - especially goldies.
You also need a liquid testing kit to monitor ammonia, nitrite and nitrate levels in your cycling and established tank. You can just use flake food as your ammonia source to feed your filter's bacteria colony. Cycling takes around a month - 6 weeks, but the only way you will know if your tank is cycled is by testing the water. An LFS will test your water for you, BUT often they will say it's ok when it isn't purely to get you to buy a fish.
Even in a cycled tank you CANNOT add 8 goldfish to your tank - you'll end up with a mini cycle as your fish produce more ammonia than your bacteria can break down. Do things slowly so that your bacteria has a chance to catch up.
As for cleaning the tank - what did you use the first time around? You must not use disinfectants or chemicals as they can poison the fish if there is a residue left behind. I use a warm salt-water solution for a couple of days.