^ Then there is what caused your problem in your fish... they are most likely getting effected by your water quality as the tank they are in was never cycled properly...
I would look up the Nitrogen Cycle in on this forum and make sure you understand how it works... coz right now your NitrAtes / NitrItes and Ammonia are probably out of control causing your fish to feel very un-comfortable...
If you were to run a fishless cycle you would need to need to feed your filter some sort of food source (such as ammonia) to get the cycle going, then as it starts to break down you build up the BB in your filters which helps support the life in your tank... so i would strongly advise brining a water sample from your tank to the store to get it tested for: NitrAtes, NitrItes and Ammonia and then let us know the results..
Also, as i mentioned before, invest in an API Liquid Master Test Kit, so that you can test your water when things seem to to south... its something you should definitely have on hand, especially when setting up a new tank...
Eitherway, best of luck with your tank, and hopefully the others don't suffer the same fate as you ray...
-EDIT-
Here is a link that explains
Fishless Cycling which may help you understand the process a bit better...