Old leaves getting brown spots, new are turning bright green

Magnesium deficiency with a developing calcium deficirency too. Generally over fed with nitrogen also.

Start a diary and fill in as much info as you can. Or state here what medium you are in, temps , feed, ph, EC, The more info people have the better your answer will be. Like someone already said over watering looks likely and in soil 3 days plus sometimes is the norm between feedings. But in Coco daily watering if not multiple times daily. Could be lockout from ph or EC too high where a flush could be needed. In soil that won't help overwatering. Calmag is a suggestion but if we knew you already was giving it to them then a different diagnosis would be given. To add to answers already magnesium or iron. Ionic calmag and some others contain added iron but most bases have enough. We don't know what you are feeding . Help us help you

High, you are right about the calcium deficiency. But you also overwater the plants. Let the top inch of the soil dry out completely before watering again. When you do this it does ensure the right oxygen and water ratio. The roots are now lacking oxygen and that is why the leaves hang so sad.

I think you answered your own question. I think the addition of calmag would be of great benefit to your regiment if you aren't already using it. Root rot also look like it's a possibility based on plant appearance. A lack of Sulphur will also turn that plant pale all over, but thats a distant second. I think Calmag will resolve the bulk of the issues it's showing, and a root enzyme like Hygrozyme will sort that root rot problem out in a matter of 7 days.