I quite like Annerley Aquarium in Greenslopes, and Smiths Aquarium in Boondall.
There's also Atlas Aquarium, and Pauls Aquarium at Oxley who are more fresh focused than salt.
Gallery Aquatica used to be at Salisbury but I think they shut that one and now just have Tingalpa. They're more salt focused than fresh though. I don't like salt. Gets all crusty.
MAD at Underwood is a pretty good store to go look around, but I found them really expensive. The store is also super busy on a weekend so I tend to avoid it.
There was a kind of dodgy looking fish shop nearby too called Exotic Fish Connections that is like... basically unadvertised? There was a single flat sign out the front and the place always looked completely deserted. Every other shop around it was closed so the carparks are all empty. And the sliding door didnt work properly so it felt like it was locked when you first tried it. We only stopped because I saw some people walk out with fishies one day so we pulled in. It had a surprisingly large amount of stuff inside and was cheap from what I remember. I think it's gone now though the website hasn't been updated to say they're gone. Possibly they were renovating? The place looked completely gutted when I last drove past. Never got fish from there, but bought some sweet looking driftwood I used in a tank recently:
