What you are seeing is not related to the temperature of the room. Though a little high, the problem you're seeing is obviously related to the reservoir.
There is definitely something reservoir related going on, possibly trace elements.
Try just tap water?
RO lacks just about everything hence the lower PPM. So you should probably be using something for trace elements.
We need more info about your res. though..

Air stone?
Is it home made?
Do you know the temp of your res?
Can you see roots? If so, are they healthy?

Hope this helps,
Bob.