Sickly Dracaena marginata (indoors)

I have a dracaena marginata. Normally, it had long slender green leaves with maroon edges.
I had it in a low-light living room and it didn't seem to be doing to well to me. It's leaves were drooping and there were quite a few red, dying leaves. Also, it had fewer leaves than healthier ones I've seen. I moved it to another room that gets a good amount of indirect light from the southwest. In the past few days, it's been about 20 degrees hotter than usual and brighter than usual. Now the draecana is doing worse. It has even fewer leaves and a lot are getting yellow, than brown-red and dying. The new leaves that sprout out of the top are dying right away.
I water this plant whenever the top inch of soil is dry.
What can I do to improve the health of this plant? Is it having a hard time adapting to more light? Is it having a hard time dealing with the changing temperature (90 rather than 70)? Too much water? Too little water?
Also, when does a dracaena marginata get to the point where it no longer can be salvaged? How many leaves does it need to stay alive? Can it come back?
Thanks
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You may just have starved it by keeping it too long in a low light environment. 20 days is nothing, it would have keeled over regardless. It is part of having houseplants - you will kill a few trying to determine how low a light they tolerate. I killed one corn plant that way - also a dracaena. Now I make sure they get good energy reserves by leaving them outside for the summer, with some morning sun.
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Thanks for your response. It is helpful.
I've had this plant for three or four months.
Are spider mites visible to the human eye?
At what point do you think I should cut top of the plant off? Right now there are some decent leaves (more than I would want to count). Would it be safe to wait until there were enough leaves to count on a hand and then go drastic?
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I have a Dracaena Marginata. It was a very small plant 5 years ago. It is now 3 feet tall and has never had direct sun, but indirect bright light. During that time it has been transplanted three times. I water it twice a month--keep it mostly on the dry side. They do not like cold. Check for insect damage and inspect the root system--if these are okay, give it a shower to clean the leaves.
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