The Marantaceae family, also known as the Arrowroot family, contains the Calatheas, Marantas and Ctenanthes - what I like to call the divas of the plant world. They can be tricky to get right but it is totally worth the effort because within this family are some of the most beautiful plants you can have in your home.
Care for the three members of the Arrowroot family are pretty similar so these tips will work for any Calathea, Maranta or Ctenanthe you have, but I will refer to them as Calatheas from here on out for ease.
Calatheas prefer medium indirect light, avoid any direct sunlight because this will burn the leaves. They can tolerate low light conditions as well if you are looking for a plant for a darker corner of your house. If your calathea starts to lose some vibrancy in it's leaves or is looking a bit sad, it might be because you have it in direct sunlight, try moving it out of the sun. (This doesn't mean they are an option for a room with little to no light, no plant is happy without any light at all.)
This is where this family of plants get their diva name, they prefer filtered water or rain water ideally. This isn't totally necessary, lots of people don't bother and their plants cope fine. But if you want to avoid crispy edges and keep the plants happy - its worth using a brita filter or collecting rain water.
Alternatively, if this is too difficult for you, you can leave your tap water out for a day or so which allows some of the chemicals to evaporate from the water before you water them.
Calatheas like their soil for be evenly moist all the time, but not too soggy. Water when the top of the soil feels dry but always allow the excess water to drain away so they aren't left standing in water.
Calatheas prefer high humidity, they naturally grow on the jungle floor where it is very humid. To increase humidity you can mist the plant regularly, keep it in a humid room like your bathroom, use a humidifier or place the plant on a tray of wet pebbles.
Fertilising your calatheas is not essential but just for the best and healthiest growth, it is good to fertilise in spring and summer or during the plants active growth period. I would personally feed the plant around once a month. You can purchase plant food here.
Pearls of wisdom
It is almost inevitable that your calatheas will get some crispy edge or leaves, don't be afraid to trim the edges off if you don't like how they look, or even prune off whole leaves that don't look healthy.