how long has it been since I've had a fishtank? Mid-80s, so 25 years? But I still remember the protocol - float the bag of fish for a while to equalize the water temperature, then gradually add tank water to the bag to equalize the pH and other content.
Truth is, I still don't have a fish tank, and I doubt I'll be getting one any time soon, but my dragonfly pond (3' diameter, 8" deep kiddie pool) has dark green water in it. A few weeks ago I buried it in the shade along the creek bank. Today I added more big rocks and topped off the water level, and... added a couple of plecostamus! They are small, about an inch long. Plecos are like carp/goldfish, in that they'll grow to optimal size for their surroundings, and no larger.
The first time I added pond water to the bag they both acted excited and happy. I noticed a teeny-tiny tadpole in the bag, less than 1/8 inch long. Now they're all back in the pond, and the bag is empty. I'm looking forward to seeing what changes they make in the water quality.