I would cut a piece of black acrylic. seal it with super glue. it can be done under water.
Yes!
or
there is a gap between the false wall and the overflow glass wall,isn't it?
I would stuff up the bottom of the gap with filter Bio sponge so that the sand will not go through.
how deep of a sanded? if its deep enough to cover the grates, then i don't think you'd need to seal it off. the sand will do it for you and still allow pods and what not to enter to feed off the detritus that collects behind the wall.
i'm in taipei and i'll pm you my line.
I don't consider this as DSB。It is not deep at all. Take my tank as example,I have got 2~3cm sand. It has been 2 years now.
I never flip over the sand as my sand is very fine. My sand star fishes is doing all the maintainence job. This might not be good for others. I have not got any trouble till now!
Why I only placed 2cm sand? because I have same concern .