hi there danotto, there are a couple of problems with your stocking which may have led to your fish deaths.
the tank is 6 US Gallons, the general rule of thumb for stocking is 1" of fish per US Gallon so you're looking to have a maximum of 6" of fish in there. what you have had in there is the following
2 x platys - 2" each = 4" total
1 x albino cory - 2"
1 x guppy - 1"
3 x neon tetra - 1.5" each = 4.5" total
1 x dwarf gourami - 3"
1 x betta - 3"
so that's a total of 17.5" of fish in a tank that really only has room for 6", you also need to consider the individual fish's requirements, only the guppies are small enough for that tank, all the other fish really need more swimming space. the cory's and tetra's need to be kept in groups of 6 or more. and the siamese fighting fish (betta) is not called that for no reason, they need a tank by themselves and particularly not with any long finned tank mates like guppies.
in new tanks as well it's common for there to be problems with the chemical levels in the tank due to a process called cycling. do you know anything about this process? if not read the link in my signature 'whats cycling' which will explain the process to you. essentially fish produce waste in the form of ammonia which is toxic to them, in a fish tank the way this is removed is by a bacteria species that lives in the filter and converts this ammonia firstly to nitrite and then to nitrate. the problem with new tanks is that these bacteria are not present in large numbers when you first set the tank up and consequently they are unable to process all the ammonia that the fish are producing. the ammonia builds up and the levels become toxic and this can kill the fish, in a tank stocked as heavily as yours is and with a new filter it's likely the ammonia would build up very quickly and we'd advise lots of water changes to rectify this. now at the 8 week mark you may be out of the woods, but it's possible that this is what has killed your fish.
do you have a test kit at all, if so can you tell us the levels for pH, ammonia, nitrite and nitrate?