Your problem is you are over-watering your plants and that's causing your gnat problem. Keeping the soil soggy or wet even for a few days in the past weeks can show up today with causalities . You never have to feed new soil out of the bag any fertilizer, because the potting mix you have has 6 months extended release fertilizer in it. I recommend you start with 3 easy to grow plants before you stock your shelf's with hard to grow plants. Salvage the plants you can, by letting the soil dry out well before watering again. Make sure you have drainage holes in your container. Your lighting is ok for most part for your plants. Good luck.
