Julie,
According to PFK's YourLocalGuide listings there are the following in Glasgow which say they do marine:
Coral Reef Aquarium, 34 Paisley Road West, Glasgow G51 1LB
Misko Aquatics, Unit 12 Block 57, Dalziel Road, Hillington Business Park, Glasgow G52 4NN or at www.miskotech.com
They also list M & R Dogfish, Glasgow 0141 427 3615 in their list of Top 40 Shops but it doesn't say whether they do marine.
HTH, Eddie