It does look like heat/light stress, but that seems very unlikely with a T5 grow light at 6" away. So you had indoor and outdoor plants, and you brought the outdoor ones inside, then this started happening?
How long did it take before you started seeing symptoms? It happened first to the outdoor plant, then it started spreading to the others? That does sound like it could be bugs. If you check with a scope at the stems, under the leaves, top and bottom, can you see anything unusual? Any spots on any of the leaves, even if they're really small? What about in the soil, or in the runoff water?
Was there anything else that changed right before you brought the outdoor plant inside (new supplement, etc) or before you started seeing symptoms? You mentioned you started spraying them, did you do that after or before you started seeing the yellowing?