I've got a Fortunella Obovata tree's leaves turning brown so I'd like to know is this some kind of diseases which would require disposing the tree or something treatable due to nutrition deficiencies. I'm (deep) watering it only when the top 10cm of the soil is dry and fertilize it in recommended dosages/intervals(Compo citrus). To me it seems as root burn due to salt accumulation but I'm not sure. Btw, the tree had lost many leaves after I had received it but it was fine for a 1.5y or so up until the last two weeks.
Over-fertilising is probably not the problem, unless you are giving it full strength fertiliser every time you are watering. Especially not a problem if you are flushing the pot every time you water.
There is small chance you are under-watering that you do not run water through it long enough, but this is slim.
I believe you are over-watering. I believe you think it is time to water, but in reality the soil is still wet. That because it appears to be free flowing it is not holding much water. That is not true when it comes to sand. Sand is a smaller particle size than both perlite and rock. It is so much smaller that it slowly works its way to the bottom of the pots. At the same time perlite works its way up to the top of the pot. Sand builds up in the bottom and traps water. Because it fits so tightly together it able to hold lots of water. When you first made your mix, it would have had lots of pockets of air, the sand will be pushed by the water down. As it is pushed down it fills in those pockets of air. A plant's roots start to decay, when deprived of oxygen. This can easily become a problem in deep pots.
You should be using something like a bamboo skewer to indicate when it is time to water. You insert it in the soil and leave it. The next time you want to water, you pull it out and look at it. If it is still wet you do not water. If it is moist, you can water. If it comes out dry you have waited too long.
The other method you can use it lifting the pot. Once you get to know the weight of the pot after it has been watered and the weight of when it needs to be watered you can get the feel for it. This method is not a fool proof as the other.
Moisture metres do not work in rocky/sandy soil. They use an electrical current that is carried by the water in the soil. The higher the current the more water there is in the soil. If there is a rock, a piece of perlite or sand in the way it will not read the situation correctly. It will get a low current, indicating it is time to water, even if the soil is soaking wet. Even the best moisture metres do not work well in this type of soil.