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One other thing that wasn't mentioned was light. changes in light can cause ficus to drop leaves... or any changes for that matter. Did you move the tree?
It doesn't look like you have mites... if there is a pest, I've seen ficus get scale (schildl?use) pretty easily, when they are inside.

Hansen wrote: It' difficult for us to give you a 100 procent correct answer. But I can give you some reasons why trees gets yelllow leaves, and then you can pick the one reason you think cause the problem. You know the trees better than us:
Under/overwatering
Chlorosis
Seasonal chance
New enviroment or too cold enviroment
Chemicals
Spider mites

Use the exclusion method.

This Sound's like a good approach, here we go:

Under/over watering - unlikely cause as: 1. mention before in a post, I have some experience with bonsai and i believe I do not make this mistake so easily. 2. out of 20 trees that all are locate in the same place, same Environment, same conditions ist the only one behaving like this.
Chlorosis - possible cause: Did not knew this term so I google it "abnormal reduction or loss of the normal green coloration of leaves of plants, typically caused by iron deficiency in lime-rich soils, or by disease or lack of light." 1. iron deficiency in lime-rich soils I use tap water wich has quite a lot of lime, although I've used a solution that should break lime. 2. disease - I'll come back to this below 3. lack of light highly doubt this as they are on a south facing windows.
Seasonal chance - unlikely cause as: 1 there is not a big change in seasons here and this has started happening a the end of august and temperature are going down just now. 2 did not move them or changed there Environment ( did not use heating systems, humidity should by relative the same )
New Environment or too cold Environment - unlikely cause same as above at seasonal change
Chemicals - possible cause as mention by Chlorosis I've been using a solution that should make hard water more soft( break lime)
Spider mites - maybe not sure: Don't have experience with bug or diseases, what i can say is that there are no sign of bug/mites, diseases do not what to say because if this was the case then the other trees should be affected as well as they all are next to each other.

My Conclusions so far:
a. I use tap water as I don't have access to rain water (i live in a flat). The tap water is hard water with quite a lot chalk(lime). Don't know if in one year of using tap water, the soil can change so fast it's proprieties and build an iron deficiency? Any Ideas?
b. I use a solution that is suppose to make the water softer, but don't know if this will still influence the soil? The solution is normally of Orchidee but the bottle was place next to the fertilizer on the bonsai section, so i presume it is safe. Since 2-3 week i stoppend using this, did not see any difference but don't know if i can expect to see a difference so fast.
c. I mentioned something about the Leaves that nobody seem to give any importance... the fact that the leaves don't just go yellow they become spotted some how , they look like rusted. If a leaf fall because of under or over water or other factor they don't behave like this...
d. All my tree(80% of them are ficusess) are grouped together but only one has this issue, shouldn't they all suffer the same normally? There is a small exception to this, two other ficuses have also lost in the same time( 2 months aprox) 4-5 leave with the same "yellow rust" look, but not like the main one which looses each week 7-8 leaves per week since two months ago.

After reading this if you have some answer to my question, please let me know.

Other then that thank you all for your responses till now!

I will try figure out exactly what the issue is and report back my findings.

When my ficus trees are losing leaves the reason often are a problem with under/overwatering or they don't get enough sunlight. Those are the reasons 9/10 times to yellow leaves. I know you said you only got the problem with 2/20 trees. But every tree are different. E.g i got 16 Elms and sometimes some of them gets yellow leaves and some don't.

I want you to think about this:

When a plant is under-watered, it reduces the number of resources sent to the least-important leaves. When you water, the plant may recover, but the damage has already been done.
Some plants react to changes in watering slower than others, so some will show symptoms of distress within a week, others can take several weeks to display signs of a problem.
Sometimes, the yellowing will show after the plant has received the proper amount of water. It can confuse you because the damage appears when the plant seems otherwise healthy. You might think the plant is still lacking water and water again, causing over-watering.

In general, if the yellow leaves on your plant are solid yellow and fall or pull away from the plant easily, you have under-watered at some point in the past. But if the leaves on your plant display stippled yellowing or mosaic yellowing, or appear yellow or light green with dark green veins, and if the leaves are still well attached to the plant, then you probably have another issue.

Try to see if you can locate the problem, with those advices.