I need basic info on goldfish

Are goldfish cannibals?
I haven't seen them eating each other but I started out
with 40 in a stock tank and am down to 9 and holding.

I wonder if I should rig up a way to aerate the water?
I have enough water running in the tank to keep it fresh
but the biggest is always at the end of the hose like it's
trying to breathe more air coming out of the fresh water.
It would be difficult to rig up a way to aerate. There's
no electricity nearby.

Any suggestions on their care would be helpful.

Thanks