This is my order of preference on this side of the bridge : HFF Mt Roskill, AquaVillage in Neilson St Onehunga, Animates Mt Eden just in case they are having a special, and they have a nice display tank. Bird Barn in Henderson has some interesting stuff too and lots of other pets.
On the other side of the bridge there's Aqua World in Glenfield, must see if you like saltwater. Then HFF Albany and Pupuke Aquarium centre in Birkenhead.
My favourites are HFF Mt Roskill for range, price and good staff, Aqua World for interesting things to see and do and some good prices, BirdBarn for same reasons, and AquaVillage as he has fry of various ages so again is nice to go there.