They died one by one over a couple of weeks. I've only had them three weeks. The tank was fully cycled when I put them in and I've checked the water every day. I do think maybe they're a bad batch of fish because surely the shrimps would've been affected too?
If it is an illness or parasite or something is there a chance it'll be in the tank still? Like if I were to get new fish how do I know whatever killed the guppies isn't still lurking?
At this point you won't know which is why what is best to do is quarantine any new fish for at least 60-90 days so that any disease they may carry should have shown itself in that timeframe. If after that quarantine period these new fish catch something once they are in your main tank, that would mean whatever they have is still active in your main tank. From there, you would have to decide whether to sanitize the tank and restart it or medicate your main tank with something that may hurt your shrimp so you would need to make accommodations for the shrimp in another tank while you are treating the main tank. The other option is to keep the tank as is for the shrimps and not add any fish to it.
