In the OP, it was mentioned that the plant was above a "rock filled water tray ... for humidity. I hope it's not sitting in its own effluent that collects in a drain saucer. Using a wick, like mike suggested, would be less effort and will accomplish the same thing as employing the pot-in-pot technique. The problem with pot-in-pot is, the water in the soil of the larger pot is going to stay there until it evaporates, and I can see some potential problems associated with anaerobic decomposition in the larger pot's soil. Skip the water meter. Buy a 1/4 or 5/16" wood dowel & sharpen it in a pencil sharpener. Use THAT to test the soil for moisture. Push it deep into the soil - if it comes out damp and dark or wet - no water required. The fertilizer is appropriate insofar as the NPK ratio, but what else does it contain? Reading the ingredient list of the potting soil and assuming the ingredients with the highest volumes are listed first, it HAS to be extremely water retentive - which means you can't water appropriately w/o risking root rot or at least impaired root function. You can't move it outdoors, but that's where it WANTS to be. Here's the thing - bottom line: The plant is obviously suffering because it's being forced to deal with cultural conditions it's not genetically capable of dealing with. If you can't isolate and fix the issues that are causing the decline, it will continue, so you'll need to go back to the drawing board and look at the big picture. The first considerations are checking for insects & root problems/disease, and making sure the plant is in an appropriate soil (if it's not, your watering habits need to be adjusted so they allow for a poor soil) and in the right light/temp range. If these issues don't come up in the favorable column, there isn't a lot of sense in chasing it much further until you're willing to take charge and make some changes. That might sound harsh, but it's just how things are. Al

Two suggestion: 1. Purchase enough coir to spread under the basil plants covering the soil completely. This prevents any fungus from splashing up on the leaves. Coir does not support fungus growth. I use this same method on my potted tomatoes and potted peppers. Urge everyone to do this on any potted plants prone to fungus infections. 2. I find that a weekly spray using what has been dubbed the Aspirin Technique stops fungal/blight problems on basil, tomatoes and peppers. Simple to make. Fill a gallon jug with water. Add two chewable baby aspirin (81 milligrams each). Allow to sit uncovered 24 hours. Pour into large container and buzz with hand blender. Pour in spray bottle. Add two drops of regular lemon detergent that does not have degreasers or antibacterial products added. Spray plants throughly in early morning or evening. Water base of plants with any left overs.

Once the leafminer larva is inside the leaf, happily mining away, there is nothing except a wasp that lays its eggs in these leafminers that will control them. As Jean stated, if you can see the larva burrowing through the leaf, squish it.

They probably came straight from a greenhouse They need to be hardened off -- in other words, slowly acclimated to the outdoor world of bright light, sun, and breezy to windy days.