Dendrobium: black leaf tips, then spots and yellowing

[Orchidtinkerer]

there are too many factors that can cause this including low temps, high temps, wrong ph, too much fertilizer, degrading substrate, too soggy substrate etc.

When looking after orchids you should be aware of their temperature tolerances depending on what variety you have, then you need to be aware of what the roots look like when you first get the orchid - this involves a repot. A good quality substrate should be on hand if the old substrate needs replacing.

Feed it rain water over tap water to avoid ph issues.

Pick large chunky Substrate that dries well and supplement it with more water retentive substrate if it dries too fast but keep it airy and slightly moist at all times over soaking wet. I'd recommend reading some basic care guides on the topic.