Organic in pots is very hard to do especially with high feeders like peppers and tomatoes.
The peppers started off strong probably because they were living off the stored energy in the seed which will get them through till they have true leaves.
I think your medium is unbalanced. All the things you are adding may not be helping since you are just guessing what the imbalance is.
I8 inches away is quite far for plants and peppers like to be around 70 degrees. However, Light issues would not cause the symptoms you describe. The peppers would be leggy and the leaves may be dark green, but they should not have dark patches on the leaves.
My suggestion is to start over.
If you want to make your own organic mix try this one that was suggested by pepper Joe
There are many commercial potting and seed starting mixes that work real well.
If you want to make your own I like to use the following mixture:
1/3 Potting Soil
1/3 Coarse Sand (soft play sand will make cement ...oh no!)
1/3 Composted cow manure
Mix all 3 parts together very well and make sure there are no lumps...to give you a well aerated mixture. Peppers like sand and it gives the roots a good chance to spread out. The cow manure has nutrients for the roots to feed on and the potting soil is nice and friable to prevent compaction. The root zone should be happy with this combination.
Great Gardening, Pepper Joe
If you want a decent organic potting mix, Gardener and Bloom has a good organic potting mix that worked for my potted tomatoes. I did add fertilizer to it.
For myself organic in pots is very hard to balance and you need to supplement. Also realize organic fertilizers are dependent on the presence of soil micro organisms to make the fertilizer available to the plants. Those microbes also need to eat so they compete with plants for nitrogen in particular. There are not many microbes in a pot to convert the organic fertilizer and organic fertilizer for the most part is not in a form the plants can immediately use.
For a potting mix and starter mix I use
2 parts peat moss (organic)
1 part perlite
a handful of vermicast or compost.
1 teaspoon slow release fertilizer per gallon of mix. (not organic- but contains readily available nutrients in slow release form so it won't burn plants. I use osmocote 16-16-16 plus. You can add 1/3 tsp of gypsum per 4 gallons of mix to add calcium and sulfur. It is more soluble than dolomite lime and my soil is already very high in magnesium. Gypsum does not alter pH. Compost that is produced aerobically is alkaline between pH 7.6-8.5 depending on source. I use the compost to make the pH a little higher. Keep in mind most potting soil mixes sold are pH adjusted with lime already added so read the label. Most plants like a pH between 5.5-6.2 except for cacti and some plants that like very alkaline conditions. Above pH 6.2 the micronutrients become increasingly more unavailable. Young growing plants have a greater need for all of the essential elements to be available than older plants. Older plants will sequestor some nutrients like potassium and tranlocate nutrients from older leaves to younger ones that need it. PH and the interaction of the elements that are complementary and atagonistic to each other keep them in balance in nature. There is nothing natural about pot culture.
https://www.alliedbotanical.com/pdf/Osmocote15912.pdf
I grow peppers in pots 1 gall to 20 inch pots. A pepper will live about 4 years in a 5 gallon pot if it is well cared for. It lives longer in the ground in frost free areas. Some of my tabasco and super chilies have lived 8-10 years in the ground.
Peppers and tomatoes are very high feeders especially in production. In pots I use vigoro citrus food 6-4-6 it contains micros. I feed peppers and tomatoes approx 1 tablespoon around the edge of the pot when first flowers appear; again with the first fruit; then monthly thereafter for the rest of the life of the plant.
The only exception is that if the plants stop production, usually from extreme heat, I will water more and try to move the pots or provide shade from the afternoon sun, but not feed them until they are able to start flowering again. Feeding stressed plants doesn't always help. It does not help to force plants to grow at a time they are struggling to support the leaves they already have.
Purplish stems and leaves are usually a sign of phosphorus deficiency but it usually does not look like that.
Peppers grown under artificial lights may have purplish leaves. The lights you have may not produce the full spectrum of light. Peppers may just need more UV than other plants. Transisitioning the plants to natural light or outdoors would probably fix that.
However, I still think all the things that have been added is probably making things more unbalanced and some of that is very hard to fix.
