Hey everyone, I'm in Manchester as well. I'll give my thoughts on the various stores I've been to.
I've been liking Pets Plus in Londonderry a lot. It's a fairly small aquatics section but they have some interesting things, stock seems to rotate pretty frequently. I've never seen a sick fish in the tanks. I particularly like the nano tanks. They also have a decent selection of rock.
Petco in Manchester is a magnet store which basically means they have a ton of options for ordering. The dept itself is very meh but they can get tons of different fish species in if you ask the aquatics specialist, and they will leave it in the bag for you to come and pick it up without it hitting their system. They were able to get me my mudskippers for half the price Zoo Creatures was selling them for.
Zoo Creatures. Some things are way way more expensive than at other stores, while some fish are reasonably priced. My advice is to know the average going price for a fish before committing to getting one from them. Maybe I'm just unlucky but every time I'm there, aquatics is staffed by someone who tells me they don't normally work in fish. I haven't felt that I've ever dealt with anyone super knowledgeable.
Fish Mike's, I agree I've never seen bettas in tanks. They're kept in bowls. I wish the saltwater had more variety, or they just get rid of some of the salt tanks and expand freshwater a little more. It's a small store and the salt tanks are always barren looking. Overall it's a decent store but I will say that Mike makes me a little uncomfortable. He's super stand offish and I'm glad to see he's hiring more people to work in the store because it's much more pleasant when someone else is working.
Jay's Aquatics is huge, I was so surprised to walk in there. Unfortunately when I was there the store was ridiculously low on fish. There were entire sections of tanks that were sitting empty. They did have some nice stuff and I got some crystal black shrimp for a good price.