There can be many reasons for yellowing leaves in peppers. One of the most common is over-watering. You didn't say anything about how you are watering. But even with young seedlings, you want to let them dry out A LITTLE BIT between waterings. Are these seedlings outdoors in the ground or indoors in pots / with lights? If indoors, best is to water from beneath - put your pots in trays and put A LITTLE water in the bottom of the tray, so the soil can wick it up. Outdoors the watering issues are trickier for little seedlings, but still you want the top of the soil to dry out a bit in between waterings.
Also, I don't know if your water in Pakistan is chlorinated. If so, it is best to put water in a bucket and let it sit 24 hrs to outgas the chlorine, so you can use non-chlorinated water for watering them.
It sounds more like they are outdoors, in which cases were the seeds planted outdoors or did you transplant your seedlings? What is your soil like? I would not keep fertilizing, especially 20-20-20 which is very concentrated. Excess fertilizer can sometimes cause yellowing also. Seedlings need very little fertilizer and it is best used half strength.
That's the best I can do with the info I have... pictures might help. Instructions for posting photos here are in New to Helpful Gardener? under Helpful Tips and Suggestions for New Members.
Welcome to the Forum and best wishes with your gardening endeavors!