You stated that you were using the following fertilizers:
- Slow release NPK: 15.3%, 4.3%, 5.9%
- General purpose NPK: 24.2%, 5.6%, 11.7%
Assuming your soil nutrients were balanced to begin with, the problem appears to be that you are using a very high-nitrogen fertilizer on peppers. Peppers, tomatoes and many other things prefer much less nitrogen than that. Most sources seem to agree that they prefer somewhat less nitrogen than phosphorus and potassium. I would probably recommend equal amounts of phosphorus and potassium, but I'm not an expert there, yet.
When nitrogen is proportionately much higher than potassium, it will inhibit the effect of potassium. Therefore, plants may get burned (like the tips of your leaves), and they may be more susceptible to disease and insects. They may also have reduced fruit size and be weaker. Nitrogen is supposed to encourage growth considerably, but in my experience with plants in the nightshade family, too much just makes the plants dwindle or have issues. However, some people report lots of green growth and little or no fruit when there is too much nitrogen (I'm guessing those people have more than window light or aren't growing indoors like I am). Plenty of sunlight really changes the way plants respond to what, and it seems to increase salt tolerance. If you could increase your sunlight, it would probably help a great deal. Grow lights may help, too.
Anyway, I hope you didn't use too much of that fertilizer.
To remedy the problem, you could add a fertilizer with no nitrogen in it (but plenty of phosphorus and potassium), such as 0-10-10 or 0-18-16, and after the imbalance is corrected, when fertilizer is needed again, use something like 10-12-12 or 18-18-21 fertilizer instead of your old fertilizer; occasionally 7-7-7 fertilizer should be good, too (especially if you need extra nitrogen). However, as I understand it, you shouldn't need to fertilize peppers a terrible lot. Some say you only need to do it twice outdoors (once when you transplant them and once right after the fruit starts forming or so), but when indoors they may do well with more frequent fertilization (but probably not that much more frequent). I'm not sure why they would need it more indoors instead of less, however. That's kind of counter-intuitive, unless it drains through the pots more. I'm guessing it depends on how water soluble your fertilizer is, among other things.
Alternatively (instead of the no-nitrogen fertilizer), you could wait until your plant uses a good amount of the fertilizer it has and then just use [10-12-12] fertilizer, or whatever you choose, when it needs fertilizer again.
However, if you really want to know how much of what kind of fertilizer to add, get your soil tested first. It may not have been balanced to begin with.
Too much potassium is possible. I don't know how much is too much, to be honest. But, if your leaves start getting yellow, you might need more nitrogen and maybe also more phosphorus.
About the insects, you might try dusting your peppers with some food grade diatomaceous earth. It's a natural insecticide. Don't get too much in the soil, though (although some is fine). Peppers don't mind diatomaceous earth in the soil as much as tomatoes do, though. Tomatoes don't really like it much, and they may wilt as a result (but may also recover eventually if it wasn't too much). Also, you might consider neem oil to protect your plants from pests (but if your plants are outdoors, you should know that it may kill bees as well as pests).