It's to your credit that you've asked us for help and advice here. You have a lovely little lemon tree, and let's hope with our help you can keep it lovely.
Potted plants need a constant supply of nutrients as well as water since they have a limited range that their roots can go. Not using fertilizer would be the same as keeping a dog or a cat in your apartment and only giving it water. It would starve. (It's ok, we won't tell
)
Lori has given you some great advice there, but I must admit I've never been able to stand the smell of fish fertilizer, even outdoors, let alone in an apartment.
