Hi Rosepetal,
Pothos do best in bright indirect light though they will survive in lower light. Because our eyes quickly adjust to available light it is sometimes difficult to assess its brightness. As a rule of thumb, low light is considered the minimum level of light required to comfortably read a newspaper. Pothos do better in brighter light. Southwest light is fine unless the
plant hangs far from the window. If you are at all uncertain that the pothos is receiving adequate light, try moving it nearer to the window but not in direct sunlight (a couple hours of late afternoon/evening sun is OK).
I think you are on the right track to suspect the problem is related to your watering regime. Pothos need to dry out moderately between waterings. Thoroughly water the entire surface of the soil until a little water runs through the drainage holes, then let the top 1/2" of soil dry out before watering thoroughly again. Using the calendar to schedule waterings is an unreliable method because there are so many variables that cause soil to dry out at different rates. A better method is to poke your finger into the soil to determine moisture.
I would also lay off the fertilizer. You are correct to be diluting it but despite what manufacturers would have us believe, houseplants require very little fertilizer and only when actively
growing. Fertilizer will not help an ailing
plant and could actually cause harm.
Do you know the diameter of the pot in inches (or centimeters)? Have you repotted the
plant since acquiring it? If so, that is also another possible cause of the pothos problems but from your description, I suspect the problem likely stems from incorrect watering.
If my suggestions make sense, give them a try and please let us know if the
plant improves or if the problems continue we'll try another approach.
