Lines 7-15 Summary

Get out the microscope, because we’re going through this poem line-by-line.

He hung a grunting weight,
battered and venerable
and homely. Here and there

So we get further illustration ("grunting weight") that the fish is just hanging there without flapping around or struggling.
In line 8 something interesting happens, Bishop gives the fish human characteristics by referring to him as venerable. "Venerable" means someone who is very much respected, especially because of wisdom, age, or character. We might expect a soldier, or rather an old war veteran to be described as "battered and venerable." Not a fish!
The speaker clearly respects the fish, but in line 9 she also calls him homely, meaning, you know, kind of unattractive.
At this point the speaker has given us a kind of a mixed bag of characterizations and feeling about the fish: on one hand he's tremendous and respected, but on the other he's kind of dead weight and ugly.

his brown skin hung in strips
like ancient wallpaper,

Bishop is getting into her poetic, descriptive mode. She creates a simile between the old fish skin and ancient wallpaper.
Notice Bishop's choice of the word "ancient" rather than plain old. It makes it seem more grand, right? All this talk of "old" goes back to the description of the fish as "venerable." Bishop is showing what she just told: the fish is a kinda ugly but totally respect-worthy creature.
Um, also, "skin hung in strips"?! Does this creep anyone else out? This fish is seriously beat up to have strips of its skin hanging off. Maybe Bishop is embellishing a bit, but given her interest in accuracy, we can assume there's at least a little skin dangling going on. Gross.
We're also finally introduced to the color of the fish: brown. This certainly supports the homely description we read about earlier.

and its pattern of darker brown
was like wallpaper:

Bishop is back into straight-up mode. Her description is plainly stated.
She's also doing her adjustment move again. In line 11 she said the skin hung like wallpaper and now in line 13 she tweaks that a bit to say the pattern, too, is just like wallpaper. She won't leave any stone unturned.
The speaker also further elaborates on the color of the fish. From line 10 we know he's brown; now we find out that he's brown with a darker brown pattern. Hmm. Still sounds pretty homely.

shapes like full-blown roses
stained and lost through age.

Roses continue the description with the patterned wallpaper. If you think about ancient wallpaper, it's likely you'll think of some sort of floral pattern (maybe something like this).
What is a little strange is how the speaker ties something traditionally beautiful (roses) to this homely, beat up, shredded-skin, old fish.
By using the word "age," Bishop reminds us once again of the old age of the fish. It seems she's really pushing the wise, old man (fish) association.