After today's test the dividers proved to be waterproof.

This is the result of the hiding of one of the dividers.





As you can see i still have a lot of work over it. 1st to cut the extra foam, then to cover it with silicone and stick sand/small stones or both to it. And to do the same for the right side ofc. :P

Btw i think to disband the PVC pipe tree roots idea, because they appeared too big for the size of the tank. The wood you see in the pics above looks a bit like a stump and ill just decorate it with ropes made by that method mentioned in the beginning of the thread.