Aerate the water with a fish pump to help the chlorine evaporate and keep your water fresh. It could be a deficiency as well but doesn't look too bad. I'd add a teaspoon of epsom salt to your next gallon of water and give them a diluted foliar spray to speed up the nutrients to the plant, if you don't notice symptoms on your new growth then thats your problem. Cal/Mag deficiency can cause rust spots as well, I just get over the counter vitamin suppliments(cal/mag/zinc) and crush up one vitaman per plant and add it to the water.