I'd say it's just reacting to completely drying out then. They don't like to dry out completely between watering, but maintain a certain degree of moistness. I'd keep water in it your humidity tray constantly. I had to buy a larger humidity tray because my smaller one required me to refill it almost daily.... that got annoying lol. Don't worry, like I said, it'll push out new growth before you know it and eventually those older ugly leaves will die off (allow them to die off on their own, don't cut them off) over time. No leaf last forever, even in the wild. Planto

Invest in a small humidifer. I use 2 humidifers, trays like Larry described, an indoor fountain, daily misting and weekly showering, 'foliage.' Sometimes I place water in a pot and let it boil until windows are steamed or dh comes home. lol. You don't have to go as far as I do, however humidity is good for people, pets and plants..much better than an arid environment. If you have other humidity-loving plants think about adding a humidifer. BTW, your Calathea is Roseo-Picta..RP's are the fussiest of Caltheas. Toni

Check the underside of the leaves. By what I see in the in the pics (whitish stippling on the upper surface) I bet you'll find tiny white bumps, like grains of very find salt on the undersides which is the classic sign of spider mite infestation and the sudden deterioration and leaf browning also go with the territory.

Your plants' issues, relative to the spoiled foliage, are most likely related to over-watering, a high level of dissolved solids (salts) in the soil solution, or an ammonium toxicity that develops when roots are asked to live in saturated soils. I'd suggest that you start using a 'tell' to determine when your plants actually NEED water, and only then supply it. A 5/16" wooden dowel rod, ends sharpened in a pencil sharpener, makes a good 'tell. Read more below, and please forgive the formatting. It's from something I wrote and saved. The leaf shedding can be from a decrease in light during the last 6 weeks or so, from nutritional issues; it can be a drought response to a high level of dissolved solids in the soil solution, under-watering, or over-watering. It also appears there might be some mechanical damage (like a bruise would be to a human or animal) and some oedema symptoms on the calathea. Using a 'tell' Over-watering saps vitality and is one of the most common plant assassins, so learning to avoid it is worth the small effort. Plants make and store their own energy source – photosynthate - (sugar/glucose). Functioning roots need energy to drive their metabolic processes, and in order to get it, they use oxygen to burn (oxidize) their food. From this, we can see that terrestrial plants need air (oxygen) in the soil to drive root function. Many off-the-shelf soils hold too much water and not enough air to support good root health, which is a prerequisite to a healthy plant. Watering in small sips leads to a build-up of dissolved solids (salts) in the soil, which limits a plant's ability to absorb water – so watering in sips simply moves us to the other horn of a dilemma. It creates another problem that requires resolution. Better, would be to simply adopt a soil that drains well enough to allow watering to beyond the saturation point, so we're flushing the soil of accumulating dissolved solids whenever we water; this, w/o the plant being forced to pay a tax in the form of reduced vitality, due to prolong periods of soil saturation. Sometimes, though, that's not a course we can immediately steer, which makes controlling how often we water a very important factor. In many cases, we can judge whether or not a planting needs watering by hefting the pot. This is especially true if the pot is made from light material, like plastic, but doesn't work (as) well when the pot is made from heavier material, like clay, or when the size/weight of the pot precludes grabbing it with one hand to judge its weight and gauge the need for water. Fingers stuck an inch or two into the soil work ok for shallow pots, but not for deep pots. Deep pots might have 3 or more inches of soil that feels totally dry, while the lower several inches of the soil is 100% saturated. Obviously, the lack of oxygen in the root zone situation can wreak havoc with root health and cause the loss of a very notable measure of your plant's potential. Inexpensive watering meters don't even measure moisture levels, they measure electrical conductivity. Clean the tip and insert it into a cup of distilled water and witness the fact it reads 'DRY'. One of the most reliable methods of checking a planting's need for water is using a 'tell'. You can use a bamboo skewer in a pinch, but a wooden dowel rod of about 5/16” (75-85mm) would work better. They usually come 48” (120cm) long and can usually be cut in half and serve as a pair. Sharpen all 4 ends in a pencil sharpener and slightly blunt the tip so it's about the diameter of the head on a straight pin. Push the wooden tell deep into the soil. Don't worry, it won't harm the root system. If the plant is quite root-bound, you might need to try several places until you find one where you can push it all the way to the pot's bottom. Leave it a few seconds, then withdraw it and inspect the tip for moisture. For most plantings, withhold water until the tell comes out dry or nearly so. If you see signs of wilting, adjust the interval between waterings so drought stress isn't a recurring issue. You might find this helpful. Al