The last thread about this place was about 7 years ago so I think it's time for a new one. It's located right in Egleston Square in JP, on Washington St. just before the intersection with Columbus Ave.
There's a fresh fish counter, but most of the action is at the fried fish counter, where they sell various takeout platters of fried seafood and french fries. I haven't tried those yet (though I plan to); what I have tried are the ridiculously cheap fish sandwiches. The whiting sandwich is about $2.75; the luxury option, haddock, is $4.00. (Plus 30 cents for cheese, which I tried just to confirm that American cheese does not improve fried fish. Indeed it does not.)
It's a pretty damn good sandwich! Served on "whole wheat" Sunbeam-esque bread, which does not interfere with the fried-to-order fish. I've tried both the whiting (aka pollock, I think) and the haddock, and without having them side-by-side I'd be hard-pressed to state a preference - both are freshly fried, nice and crisp, and go nicely with the tartar sauce and hot sauce they provide at the counter. The fry job is excellent and just thinking about it is making me eager to try the full seafood platter.
The fish market side has had a very limited but fresh-looking selection when I've looked - usually some salmon, haddock, pollock, and porgy, which they'll cut/filet/etc. to order.