here's the picture

Hi & welcome to Gardenweb! I would encourage you to visit the house plant forum, which is much more busy.

My balcony is facing east-north, they get strong morning sun daily, sometimes can be quite hot.
This may be too much sun. The leaves look sunburned, but am also concerned about what sounds like overuse of fertilizer.

I water them with tap water every 5 days by flooding the soil.
This may be too much, or too little. It looks like the pots do not have drain holes in the bottom. Either way, it's not possible to tell if 5 days is a good amount of time. Plants do not always use water at the same rate, so a set schedule can be more hurtful than helpful. Water should be added just before the soil gets completely dry and no longer moist. Flooding the soil is good if the excess water has a hole to get out of the pot. Would you be able to trap some rain water for your plants? It's much more healthy than tap.

I spray a bit fertilizer mixed with water on the leaves every night. This sounds like WAY too much. The label should have instructions. Using about half of what it says is usually the safest thing to do. Plants die extremely rarely (like never) from lack of fertlizer, but too much can definitely be detrimental or even fatal.

Sometimes I pour unfinished soup into the soil and hoping that it will fertilize better. Your pot is not a compost pile. This could also attract pests to your soil.

My in-law just leave them outdoor and they were good, now with routine care they're dying, are they infected by diseases? The plants in the ground have roots that are not constrained by a pot and the adverse conditions discussed above.

I really doubt my talent in gardening now. Having a wrong idea or two doesn't mean you have no talent. Just means there's some things to learn. Glad you're here trying to do just that. Have you asked your relatives about these things? Hope that helps a bit!

Also re: the soup-- you're probably salting the soil and burning the plant's roots. :(