your fish probably died from your tank not being cycled, (poor water perameters). I would do what everyone else pretty much says, get a master api water testing kit. Keep the fish you have now, and do weekly water changes (40%-50%). The fish you started out with are basically your cycle-in fish (even though fish lovers recommend doing fishless cycles because you risk losing fish in the process). Any how, if your water looks okay, but fish are dying its more than likely the stress they are going through due to your water conditions. Hopefully if you keep up the water changes, the fish that survive over the first 1-2 months will have cycled your tank, then you're good to go. Still have to do water changes though, and get a bigger tank eventually unless your getting rid of fish. Now getting a new tank and cycling might be a while, but that process can be sped up somewhat if you transfer the filter media, etc from your current tank to the new.