Me and my brother turn the fishes home to his doom, for when we work together we take his breath away, one of us is an unassuming treat, alone we do nothing but together we make a mess
This is referring to fishing with coke and mentos. Mentos is an "unassuming treat", but when coke and mentos are put together, they give off CO2, which causes "a mess", and "takes [the fish's] breath away".
A worm and a hook?
Me and my brother turn the fishes home to his doom,
They’re used in conjunction to catch fish
for when we work together we take his breath away,
Ostensibly when you catch fish you take them out of water
one of us is an unassuming treat,
The worm is
alone we do nothing
A worm by itself doesn’t catch a fish, and neither does an unbaited hook
but together we make a mess
Catching the fish
I would say nutrients and plankton, which together cause algal blooms that deoxygenate the water and kill all the fish: https://en.wikipedia.org/wiki/Eutrophication