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.

Strangely enough, some soils seem to drain quickly, but still hold too much water, and how much water the soil holds between the soil particles is a PRIMARY factor in determining what sort of opportunity the soil offers in terms of the plant's ability to realize as much of its genetic potential as possible. Your plants are programmed such that if you do your part, the plant will always look stellar. The problem is, how to figure out what 'your part' is. Briefly, the growers only job is to figure out what's stopping the plant from realizing all of its potential, and then fixing it. The biggest problem most growers deal with is an inappropriate-because-it's-much-too-water-retentive soil, followed closely by an inability to provide as much light as the plant needs. Some soils, right from the bag, will support 4-6" of perched water, which means that when you've watered correctly - so you've saturated the soil completely and at least 15-20% of the total volume of water applied has exited the drain hole(s) - 4-6" of soil at the bottom of the pot is 100% saturated. Obviously, anything you can do to reduce the volume of saturated soil will have an impact on root health and the ability of roots to function as they should. This info can help eliminate a lot of the limitations associated with excessive water retention. Most growers don't realize that a reduction in photo load can cause foliage of plants perfectly healthy at a given level of fertility and grown at higher light levels to show symptoms of fertilizer burn when moved to lower light levels. The closer to being ideal cultural conditions are, the greater fertility the plant will tolerate/appreciate. I also flush plants at the first opportunity after I buy them, for the reason I just mentioned and for the fact that I like to be in control of what my plants get and when they get it. Al

The link will take you to a holistic Overview of Good Growing Practices, which has a strong emphasis on grow media why a grower's choice of a growing medium can have so much influence on how rewarding the growing experience turns out. Also, I wouldn't be too concerned about mold as it relates to the plant's well-being. Plants live in perfect harmony with thousands of species of fungi with only a few capable of causing infections with potential to jeopardize the plant's viability. Al

I believe the plant is suffering from too much water. I suggest you repot it while there's still a chance: remove the soil, wash the roots, and repot in new, barely damp soil, consisting of a mix of equal amounts regular potting soil and cactus soil. Actually, I recommend searching this site for 5-1-1 soil formula; but, if you can't procure that right away, the mix I mentioned should be fine for now. In any case, try not to use any of the existing soil, just in case it harbors any pests. Be careful that the pot is not too big for the roots. Particularly if there's been some root loss due to rotting, you plant may not need as large a pot as before. Last but not least, keep it in a draft-free location with indirect light, and try to find out what the relative humidity level is around the plant - anything below 50% would probably be insufficient for this plant.