Certified GKH Gardening Expert
Mature plants that are fruiting will require something that is high in phosphorus, with an intermediate potassium amount, and low nitrogen.
"Bloom Boosters", as they are called, will contain just those proper ratios. You will want to apply these at 1/2 or 1/4 strength to start out. This is to ensure that the plants respond well to the nutrients.
This article will help you to grow Jalapenos: https://www.gardeningknowhow.com/edible/vegetables/pepper/growing-jalapeno-peppers.htm