Soil Temps...Hard Water

Soil temp below 60 degrees F. Water is 780ppm from tap...What are my options?

I have four plants in 5 gal. buckets.

1 is on the floor and the other three are raised a foot off the floor.. The difference in temperature is 20 degrees from the bottom of the bucket on the floor to the mid to upper canopy tops. Could this be affecting my nutrient intake to the cold plant? It is showing severe canoeing of the leaves as well as bronze color spreading evenly thru the leaf fingers. The other three plants are perfect.

Also, my water is 780ppm at the tap..after filtering, it is 580.. Is there anything I can do besides reverse osmosis or buying distilled water...

I should say that they are in third week of flower and a total of 9 weeks old.. This just happened as the temps outside here have fallen to around 40 degrees F.. It is making the plant look like a potassium deficiency but the nutes has been the same for all plants..everything is identical except for the height off the floor on that one plant.. Any ideas?