As stemie said, this is likely partly caused by overwatering. What struck me most in your question was the statement, "filled with a mix of about 50% topsoil and 50% sand". This is completely unsuitable for potted peppers. No wonder it is showing chlorosis. Always use designated potting soil, or follow the basic guidelines for building your own.
And when you are expecting a plant to produce, you should fertilize it. The easy way is to use a commercial water soluble balanced fertilizer, and give it with the water once every two weeks. Don't use any higher a rate than is specified by the product label, or you may harm/kill the plant. Or, you can try to be natural, and use compost tea, a side dressing of compost, or other natural fertilizers (see this answer for more info). Either way, you'll be helping the plant make food to support the production required of it.
On the watering, you have to watch the moisture level in a large pot carefully, as overwatering is very easy. Never water unless the top ½" of potting mix is dry. Then water thoroughly (until water runs from the drainage holes). Too wet of a condition, even if it doesn't give rot pathogens a foothold, will cause the plant to have serious issues taking up nutrient, and there will be a huge lack of oxygen in the root system. That alone can kill a plant in time.
I would suggest repotting into a suitable mix, which can be pre-fertilized, or you can fertilize yourself. Try to clean the sand/soil from the roots as well as possible, while leaving them intact. It the mix in the previous pot was suitable, leave as much as possible in the root ball. Make sure they are well spread in the new mix, and water the first time until damp, not soaking wet. Expect a little leaf drop from the transplant shock.