Change of subject! Here we thought we were going to be talking about the Civil War, and Lowell opens with mention of an aquarium.
Lowell establishes place (the South Boston Aquarium) and at least a general sense of the present (he writes in present tense, and an aquarium is more or less a modern-day invention, not part of the Civil War era).
So we should take note that, with this first line, Lowell not only introduces an unexpected subject, but also places us in an unexpected period of time.
in a Sahara of snow now. Its broken windows are boarded.
We get more explanation of the scene—it's winter. The Aquarium is totally shut down, and presumably has been for a while. It's not only boarded up, but the old windows are broken.
It's a pretty sad and desolate sight. It's as if it's been completely forgotten.
In these first two lines, notice how Lowell creates a picture of desolation using these two opposing places: South Boston and the Sahara. Boston is nowhere near Africa (where you can find the Sahara desert), and it certainly doesn't snow in the heat of that desert, but somehow the mash-up works and we get it: it's a forgotten wasteland.
The bronze weathervane cod has lost half its scales. The airy tanks are dry.
We're getting further imagery of the old Aquarium and evidence of decay. This is no longer a place you'd want to take kids on a fieldtrip… unless you wanted to seriously bum them out.
Here the fish (cod) seems out of place blowing in the wind, especially given the aquarium mention—fish obviously prefer water! And the tanks, which used to hold water, are described as "airy" and "dry."
Lowell is setting up a pattern here, just like he did in the previous line, of describing things in the opposite way of what you'd expect. When you think of the Sahara, you think of extreme heat, but in the scene he describes it's snowing. The fish should be underwater, but instead it's blowing in the wind. While these are unusual descriptions, they're not inaccurate. In fact, because they're strange, they are even more attention grabbing, and help to create vivid images.