If you're giving the soil a good flush (not just letting the water dribble through) then that part should be okay. Quality of the water could be a problem. A lot of tap water is okay but sometimes when water authorities make it fit for human consumption, they end up making it unfit for plant consumption (some plants at least). Hadn't heard of seramis before so looked it up and checked the data sheet. It contains Calcium chloride (CaCl2) and Potassium chloride (KCl). The KCl is given as less than 0.5 grams per litre which seems not overly excessive. But they don't give the amount of CaCl2, only express it as giving a pH of 5.6, so an unknown quantity. I know most plants (other than Coconuts, Mangroves, Samphires, and the like) have a low tolerance of Chlorine (chlorides). As rainforest plants this would be especially so for Calatheas, but we're dealing with an unknown (CaCl2). Of course the potting mix isn't 100% seramis but that's one aspect of the situation your plants are in. It may be insignificant, but worth a thought. Your temperatures aren't a problem. Mine sit in much higher than that but are okay. Relative humidity could be a problem though. Air inside houses (especially when there's heating, either from sun getting in or artificial) has low relative humidity. That could explain why the leaves are curling up like that.

Hi, I know this is an old thread but I think I have accidentally propagated a calathea by cutting! I was pruning back some of the older, sadder branches and accidentally chopped off some pretty new ones. I decided to put them in a vase rather than throw them out with the old ones. Queue less than a week later and I have some what looks like a little root sticking out the bottom! Wild. Some pix: Just chilling in the vase. Mum plant looking fine. Tiny baby root. I don't know whether I should keep it in the water or move it to a pot. I didn't expect to grow anything so I may just keep her in water and let you know how goes.

Where do you live? Leaves tend to brown in winter with low humidity/dry air.

can you post a photo of the plant?