Answer: You are right that normally one would not fertilize an ailing plant. In this instance however it sounds as though there is a soil deficiency of some kind. In citrus, it is probably a micronutrient problem. I would suggest using a chelated iron product such as Ironite per the label instructions. This iron, manganese and zinc supplement should be given once or twice a year. Then, from spring to fall, use a complete water soluble fertilizer that includes "minors" or micronutrients per the label instructions. You could start that now. If you have had your tree for several years, or if it seems badly rootbound, you may need to repot it in fresh soil mix as well. Finally, make sure it is in as bright a location as possible at a cool room temperature and in ample humidity, and make sure you are not overwatering it. Here is an article on container citrus care you may find helpful.
Good luck with your tree!
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