I would watch the paradise fish for aggression with the rams &, as jake said, with each other.
How long have you had all the different fish? Any new additions lately? What size are the fish? Still young & smallish or adults? How many rams did you get that you're down to 3? Can you sex them yet? Rams are pretty mellow for cichlids but that doesn't mean they might not fight.
You have some fish (neons & paradise) that like low 70s. The plecos & Siamese are ok at a range of temps, say mid 70s to high 70s. Rams are happier at 79-80ish. But a few degrees shouldn't kill them or any fish fast. It's like jake said, a long term thing.
Can you post a pic? We might see something that could give a clue.
What's the parameters differences between your tap water aged 24 hours & tank? It could be as "simple" as your water company doing some spring maintenance. I have seasonal water here & where I've lived before. In a vague general way where do you live? In US or where? State, nearby big city? What test kit or strips are you using?
Rams are now bred by "fish farmers" & are not as sturdy as they used to be. Are they long finned &/or balloon fish? Electric blue? These traits are fairly easy to breed for, but it's done by inbreeding that can affect fish health & longevity. (can you tell I'm not a fan?)
Lots of questions but they can help us help you.
