Colin Lewis has an excellent book called The Bonsai Survival Manual. Its section on helping sick trees is the most complete I've seen so far. It has a section on the Fukien Tea, and it says that temperature drops may cause leaf yellowing, and that the Fukien prefers a temperature range of between 55 and 100 Farenheit, can be kept indoors in partial shade year round, so it should be fine where it is, so long as there isn't a chill draft. It likes high humidity, as does the ficus. I would suggest you pick several cheap bonsai trays (these are usually $2-5 depending on size), filling them with pea gravel, placing them under all your trees, and keeping the trays full of water at all times. The improved humidity will keep your trees happy with less root watering, giving you a more foolproof way of providing water to your trees.

If your tree still isn't happy, you can pull out the Gardener's Secret Weapon, SuperThrive, which is mostly rooting hormone and B vitamins (it was recently proven that plants can extract vitamins directly from the soil, though SuperThrive was very good empirical evidence long before that). When the plant is looking better in early spring, you can start with the suggested low-nitrogen fertilizer, which should help fill the tree back out (do NOT feed a sick plant fertilizer, unless it's definitely sick from lack of nutrients).
Good luck!