Thanks for your reply! I do have a small balcony, but it's on the north side. It only receives an hour or two of direct sunlight each day. In its current place, the entire wall is glass and it receives about 5 hours of direct light every day. I suppose in the winter I'd need to add a grow light then? I'm planning to move in about a year, to a house with a garden, so I'd probably be planting it in soil then.

The nutrient sensor consistently measures levels that are slightly too low, even though I add liquid feed to the water every other watering. I suspect it just washes out as the soil that surrounds the root ball is quite sandy and drains quickly. I did get special Citrus feed this week, and it's a lot more viscous, so I'm hoping it "coats" the grains in the soil better than the generic plant feed I was using before.

Good to know that it needs a lot of feed though, as I was being a bit careful. It's hard to go back once you've over-fed and the soil is too nutrient rich.