flouro can be with in inches of the plants. the 40 watt tubes do not get hot enough to burn the leaves if the leaf is touching it so I would keep them low myself. Every foot a lumen travels its strength is cut in half. If the fert residue was burning the leaf it would be burnt in the channel which it is not. It would not be a balanced burn on each side of the leaf either. Dont worry about mites I once had a plant I was looking after for a friend that was devistated by mites and I had to move her out side. the nugs were covered in webs and the leaves got real dry but it is even mostly and very spoted.
One love
c