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.

What temperatures and humidity are you maintaining? Calathea like it humid. Dry indoor air -- especially common for those of us in the northern regions over the winter -- is stressful to them. They like it rather warm -- temps in the 70sF or up. Keep the plant away from cool drafts or from furnace/heater vents. Having chilly air or hot dry air blowing on it is not a happy situation. Watering can be an issue. Depending upon where you live, watering may need to be adjusted throughout the year. If you keep your home cooler in winter the allow the plant to get a bit dry (not parched, not bone dry) between waterings. When temps are cooler, the plant slows down (so to speak) and does not need nor make use of as much water. Keeping it too wet at such a point, can result in root rot or even stem death due to bacteria or fungi. In the heat of summer, they can use more water and so can be kept damper. As other have said, avoid direct sunlight -- bright indirect should be good. Keep in mind too, that plants require time to adjust to new conditions. How much time and whether adjusting is even realistically possible depends on how radically different your conditions are from the original growers (not the BBS you may have purchased it from -- the REAL growers ... as in the nursery that supplies the BBS) as well as the type of plant itself (some plants are simply far more delicate than others). Constantly switching conditions as you seem to be doing -- while completely understandable -- can keep the plant in a constant state of stress. Do not fertilize any more for a while! A stressed plant has enough issues to deal with. Fertilizer is not a "magic cure-all". It can actually make the situation worse. What is the plant potted in? The media in which most BBS plants are potted is usually -- to put it mildly -- absolute "crap." Typically the media is heavily or entirely peat based which means that it holds a great deal of water for a long time BUT once it dries out it tends to become hydrophobic (water repellant) meaning that it takes "forever" to remoisten. It is possible to have the outer ends/surfaces moist while the center of mass is completely dry.

Cj-Speciosa, your plant looks in Excellent shape, why bother repotting, unless its bothering you.the only oppinion that i would give you is to remember that Calatheas are slow growers and when being repoted you tend to damage the rooting without knowing it so you have to be very caeful when doing so.Remember that the calathea plants are very delicit in the rooting because they tend to get stress while repotting so keep that in mind if you do repot it, one other thing if you do make shore that the next pot is the same way glazed at the bottom like mine are because the other ones tend to absorbe the water from the bottom. Any way Cj your good and i wish you good luck with your palnt it`s beautiful. P.S If any mor questions feel free to write back . Slickrick. one other thing that is the max grown higth for the Calathea plant

Beautiful plants Debra. We just moved to Boston at the beginning of the month. What soil do you use? I have had this gal for maybe 3 years. I've actually thought about throwing it out a couple times when it had maybe 5 leaves and they were all brown then I found this forum. Over the summer it has been growing like crazy and the leaves have not been turning brown, the brown ones in the middle are very old leaves, new growth all on the outside.