My wife recently fell in love with a lovely Ficus Robusta/Elastica plant and bought it for her studio. Now, she loves looking at plants but doesn't like caring for them! Haha. So that responsibility falls to me. She bought it from a reputable seller and it came in good health (as far as I could tell), though some of its leaves did display some rough treatment, I think.

Anyway, it generally seems happy at home. We live in Madrid, Spain, which is a 9a Hardiness Zone, and it's very, very dry (25% in July, when it's the driest, and 60% in December, when it's the most humid). I was told to let the earth dry completely before watering this plant. Then, I investigated a bit and found a recommendation to completely soak the earth when watering it, and allow the excess to naturally exit down the bottom of the pot. I have a humidity meter, which I stick into he earth, all the way down to the roots, and I make sure it stays within the "moist" area; whenever it goes into "dry" and I corroborate by feeling the earth that it is in fact dry I water this plant. I don't completely soak it, as my instinct tells me this can't be good, but I do give it a copious amount of water.

Now, it seems to be growing fine. Since we have it, many new leaves have sprouted and they seem healthy (you can see the first couple of new leaves at the very top in the first 2 photos). But some older leaves, especially down the bottom, display brown borders, or they start getting yellow spots, and then turn almost completely yellow and drop off... And I don't know why. On the one hand, I read that the leaves on this plant turn yellow due to over-watering; on the other, they say the borders become dry, brown and brittle due to lack of sufficient water... So what is it! Haha.

I've also noticed that the leaves are very delicately connected to the trunks, because even when trying to delicately move around the trapped leaves to give the plant a better form, some leaves that seem completely healthy just snap off with the least force being applied. In fact, as of yesterday, two apparently perfectly healthy leaves have fallen off. I don't know if this is normal for this plant...

In any case, I'm starting to see it having bare patches here and there, and that no longer seems to me like the plant is too happy.

Now, when I'm watering it I'm actually doing so with drinkable water from the supermarket, as I've read that it may not like tap water.

Another detail is that our apartment is really rather dark. I try to move this plant to the kitchen (the only room where we get some nice sunlight some hours a days) as often as I can, so it can get better light. Could it be a lack of light that's generating these issues? (I'm also looking for a good UV LED plant light to help due to the lack of sunlight—can anyone recommend any?)

Anyway, I'm writing with some desperation now as I'm seeing more leaves turn either yellow or brown and fall off, so I'm getting the impression that whatever is happening is getting somewhat worse... Then again, you might tell me this is natural, that this plant is like this, or that it's simply getting accustomed to its new home.

Thank you in advance for your help!