I'm just trying to wrap my head around this. Was gonna stay away, but, doesn't the down tube have a grill on it? These would have to be pretty small fish to be getting sucked through the 1~2mm gaps. Not to mention the fishes inability to swim away.
I only say this cause I have a FX4 on my 75 gallon. Insane flow rate for that tank. Doesn't even phase the fish, even when they were little. Has a huge intake screen. No foam.
I have an Emperor 400 on my 30 gallon. The cory in there swims by with no issues. That thing has some good suction too. More than needed for that tank. Even the small mollie fry in there can swim away with ease.
I just can't wrap my head around what you propose.