Matlock Bath is about a 20min drive from my house, but I hadn't visited the aquarium for around 10 years. This is because it's not even really worth the 20min drive!
It's a diverting enough half an hour for the less-than costly £2.40 entry fee, padded out with a small geological display and a hologram gallery, but the fact remains if you want to see a diverse collection of fish you'd do far better to head to the aquatics and reptile shop at Pinxton (Wharf Aquatics), which is superb.
Where Matlock Bath does very good work is in the rescue side - many of its animals are ex-pets that outgrew their home.