Usually the expression is "go down the tubes", but "down the tubes" is the operative phrase. For example, when we flush a toilet, everything goes down the tubes or pipes, never to be seen again. It's all gone. So if an aptitude for math and sciences were wasted, we might say that it had fallen down the tubes.

It's an unusual one. I'd hate to see it fall down the toilet. That one is similar. The tubes are are the "rubbish tubes" that you find in high rise apartment blocks.