can we see a broader pic of all the plants near it ..

its classic signs of etoliation.. aka.. elongation due to lack of proper light ... but one would not expect such in a south facing window.. unless the issue is the shears ....

might need to find a different spot for it.. or if they are all suffering.. maybe you will need to open the shears once in a while ...

though to our eye.. a shear might SEEM to let in enough light.. that doesnt mean its USEFUL light to a plant

finally .. is there a heat register under the window.. and is blasting this thing now that the furnace is on... see PS ...

ken

ps: BTW.. in every post.. tell us w2here you are.. big city name .. e.g. maine winter light is not like tampa winter light.. etc .... its usually not an issue with houeplants.. but in this case.. it might be ...

I'm in Massachusetts. There is a ceiling heat vent sort of near the plants but I don't have the heat on yet.

I've had the watermelon pepperomia since the Spring and it hasn't done too well. I have re potted it and moved it around to various spots for different light intensities (shady, bright indirect, etc).

I can get a small compact full spectrum light if not enough light / wave length is the problem.

Your mix is based on a large fraction of coir, which is chemically and physically different enough from peat that for best results allowances for those differences must be made. One glaring potentiality is the fact that phenolic compounds in coir are allelopathic (phytotoxic/ poison to plants) for a large number of plants.

How do you determine when it's time to water the plant, and how deep is the soil in the pot?

Al