Hello! I have 2 Calathea roseopictas. Both are suffering from brown spots with yellowing of the leaves, then leaf drop. One of the 2 has lost half it's leaves. The other still has most. I purchased them in Decemeber. I water them only when they are dry according to my water meter. I looked at the roots, they appear OK. I looked for pests and couldn't find any. Although when I pulled the plant out of its container there was one little bug that resembled a fungus knat. I have researched the web and it ranges from fungus perhaps or under watering or gas furnaces which I do have and then there is that one little knat. I have a picture of the plant but I see that I can't add it here. Any ideas?

HI! Calatheas are very prone to spider mites, bud mites, mealy bugs and aphids. They're also sensitive to flouride toxicity and low humidity can cause marginal leaf necrosis. Fungal and bacterial leaf spots are common.
Since you only found one bug but do mention the leaf spots I'd consider a fungal/bacterial problem and procede accordingly. Hope this is helpful!

Start by checking your water quality. Hard water will cause the symptoms you described. If your tap water is on the hard side, then switch to filtered, distilled or rainwater.

Allowing the soil to get too dry is also a cause of leaf spots. Moisture meters are notoriously inaccurate, especially in soils with excess minerals. Discard the meter and water your Calathea whenever the top half-inch of soil feels dry to the touch.