Making recommendations,whether right or wrong, can bring either praise or condemnation depending on how the plants does. That said, I am wondering if the tree is actually being under watered and therefore also underfed. Container grown citrus trees have never been killed or hurt by over watering. What actually damages or kills the tree is the lack of root zone oxygen, because the water fills all the air pours in the medium. However, the actual culprit causing the lack of oxygen is not the water but a medium with insufficient drainage. Some time back I conducted two experiments. Using a well draining medium with good soil aeration, I planted two different citrus cultivars. I watered one tree every day for two months, and the second tree received water twice every day for the same period of time.. At the end of the experiment both trees were strong vibrant healthy plants, and had put on good growth. For plants, it is the water stream that delivers nutrients, proteins, hormones to all parts of the plant. Cactus/palm/citrus mix already has good drainage, and with the addition of extra perlite the drainage (thus soil aeration) should be outstanding, and probably too much so. In the time that you have been growing your seedling, it should have put on more growth than it has. Anyway, good luck to you and your seedling. I know it is an important tree for you, especially as it is a Dekopon (Sumo) - Millet