drifting from the noses of the cowed, compliant fish. (8)
The fish are trapped in the aquarium, just as the slaves weren't free. The use of "cowed" and "compliant" shows us Lowell is making a connection between the behavior of the fish and the behavior slaves are forced into. We'll never see aquariums the same way again (thanks a lot, Lowell).
Behind their cage, yellow dinosaur steamshovels were grunting (13–14)
This is sorta like the fish. Lowell shows the steam shovels working from behind a cage. This hits on the trapped thing again, and introduces the idea of labor (something slaves were no strangers to). The idea of them grunting adds to the connection of the slaves (a human connection), but it's also pretty silly to picture.
He rejoices in man's lovely, peculiar power to choose life and die— (37–38)
This is a nod to the somewhat positive fate of the soldiers—that they were free men, and were free to choose to fight for their country, and to die for it. The lovely part is that they were free. The peculiar part is that once they were free they gave up their lives. Seems like a pretty steep price to pay for freedom.
and lost with his "n*****s."(52)
Racism against the black infantrymen was rampant. Lowell puts the word in quotation marks to make sure everyone knows it's not his word, but borrowed from the racist haters during the Civil War era. That they were thrown in a ditch all together gives you a sense of how little respect their enemies had for them. That is not exactly the picture of sportsmanship.
the drained faces of N**** school-children rise like balloons (60).
The faces of the children reflect African-Americans' struggle since slavery. Although they're rising (seemingly a good thing), Lowell writes that their faces look drained (presumably tired from the centuries-long fight for civil rights). This is also perhaps one of the weirdest images in the entire poem. It takes a certain amount of cartoon-watching imagination to be able to picture this one.
a savage servility (67)
Though slavery is long over, some of the same awful qualities are still alive in present-day Boston. Lowell might also be playing with the word "savage." A common racist idea of the Civil War era was that African-Americans were like savages. In mashing together "savage" and "servility," he calls to mind the perfect stereotype of a slave—long after the presumed end of slavery (and the backward thinking that excused that awful practice).