In my local pet shop there are many compartments for fish. One of them is labeled coldwater marine fish. They need salt but do not like heaters. Fish in that section include saltwater 15 spined sticklebacks, sea scorpionfish, rock goby and shore clingfish. "They are easy to care for but very dull" the shopkeeper said. They are all native to UK shores because I found them all in a 'rock pooling' book.