Like other houseplants, Calathea plants prefer rainwater. However, if you only have tap water, that can also be used.
Houseplants should be fertilized from early spring to late summer. Calatheas should be fed every month with a balanced liquid fertilizer.
Fertilizing the plant will keep it nice and healthy through the summer months.
Any problems with Calatheas are likely due to over-watering or under-watering the plant.