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38º would be pretty bad but that's air temp if it was only a few hours. The leaves will show you if they were damaged. The soil buffers the root temps, so I doubt they got that low. Mid-60's is the lowest you want to go for root temps is what I've heard. Not in terms of damage, but they don't do much nutrient uptake below that.
55º is fine as a fluke. Definitely not good as a regular nighttime thing!
Cold temps can do that. Are you up and moving?
If it's mostly chlorine you want to get rid of, 48 hours of standing out will take care of that.
I bought an RO filter and stopped using it for plants. At least my plants take to tap water really well.
Outgassing the chlorine won't drop the PPM much as it's very concentrated, so they don't need much to disinfect the water. But letting it stand will clear it.
What's in your water you want to get rid of? A number of growers stopped using RO water (0 PPM) because their plants suffered from a lack of all the good things in tap water, including loads of trace minerals along with calcium and magnesium.
My tap water comes out at 220 these days and I nute as if it's zero. When my meter reads 820 I just know that means 600 PPM of nutes.