Police are investigating a suspected poisoning attack that killed hundreds of tropical fish in a newly-opened pet shop.

Around 300 fish were killed after a thick 'carpet of food' was poured into six tanks at Paws pet shop in Musselburgh, East Lothian.

Staff believe a suspicious couple who were left unattended may have deliberately carried out the sabotage.

Shop owner Sam Lawson, 24, faced the 'heartbreaking' task of spending six hours removing all the dead fish.

She said: "You wonder what goes through people's minds.

"Even if someone had a problem with me, there is no reason to hurt the animals. This has been an attack on Paws. 

"Whoever did this, it took thought and planning as they were in and out in a matter of minutes."

Miss Lawson said that the tanks were laced with toxic liquid and elevated levels of ammonia were detected in the water.

Staff claimed the water had turned an unusual orange colour and many of the dead fish were tropical cichlids worth up to £20 each.

Dr Martin Brammah, a tropical fish expert, said "an ammonia spike in the water" was one of many possible causes.

He added: "If you wanted to poison them you could do it quite easily.

"If you have too many fish and then more food is added, ammonia is produced. It could have been a problem with the water supply, or even a lose wire. 

"If someone poured something like a can of Irn-Bru in that could easily do it, particularly if the tanks are on a centralised system."

Jenny Quirk, 30, had been supervising the shop while Miss Lawson spent her birthday with her family.

She believes that an older man and a younger woman went into the aquarium while she was busy dealing with customers in the front of the shop.

A Police Scotland spokesman confirmed a probe had been launched and called for witnesses to come forward.

He said: "Police Scotland officers responded to a complaint from the owner of a pet shop that someone poisoned most of the tropical fish in the shop by putting excess food into the tanks. Enquiries are ongoing."