Looking at your pictures, I don't see any scale, or bugs of any kind. I also don't see the symptoms of a nutrient deficiency. Therefore, the problem must be inside the container. Citrus and cactus soil is not the best medium to grow a citrus in, despite the name of the product. If your tree has been in that soil for a year and a half, I believe that the medium has compacted to the point that the roots are starting to be oxygen deprived. The addition of turface or fir bark chips to such a mix would lighten the medium and would correct this. I totally agree with Laaz, that a medium either of turface mixed with Miracle Grow Garden Soil (MGGS) would be a much better medium to grow your tree in, or the traditional 5-1-1- mix. One word of caution: not all MGGS is the same in all sections of the country. Examine the MGGS to insure that it contains a lot of wood chips before you purchase it. Whatever, you need to slip the tree out of the container and examine the trees root structure. See how much water if being retained and the porosity of the medium.