Did El Diablo the lonely shark really take a chunk out of Friday the turtle?

EL Diablo is innocent! Justice for el Diablo! Free the Fishtank Felon now! The 5ft-long nurse shark at Devon’s National Marine Aquarium has a bad reputation. he’s the hooligan of the aquatic world, already expelled from two sealife tourist attractions — and now he’s the chief suspect in an unexplained attack on a turtle.

Curator Marcus Williams, guiding a film crew around the giant saltwater tanks in Plymouth on Secrets Of The Aquarium (BBC2), admitted el Diablo (Spanish for ‘the Devil’) was on his final warning. ‘he was a very good shark for years,’ he said, ‘but unfortunately his mate died. And at that point, he became a bit of a monster.’

I didn’t know such a thing as nurse sharks existed, but this one is clearly more Nurse ratched than Nurse Nightingale. To make it worse, the mysteriously injured turtle was a favourite with the public. Friday is a green turtle, the species seen starring in spectaculars such as Planet earth III. honestly, el Diablo couldn’t have made himself more unpopular if he’d taken a bite out of Sir David Attenborough himself.

But even a shark deserves a fair trial. This entertaining and lighthearted six- part documentary

shows how much planning goes into looking after the denizens of Plymouth’s deeps.

When el Diablo was moved from one tank to another, a team of about 20 scientists, vets and assistants were enlisted, to sedate him and wrap him in plastic sheeting like a giant sushi takeaway. Then he was transported in a stretcher, up stairs and along corridors, to his new home.

Friday proved even harder to move. As he was lifted from the water to have his wounds inspected, he lashed out with a flipper and sent a vet sprawling. James herriot had his challenges, but he was never knocked over by a turtle.

Not all the aquarium’s inhabitants were so grumpy. Marcus went diving with a giant grouper, a sort of sulky-looking sea bass from the Indian Ocean, with the temperament of an affectionate Labrador.

Groupers enjoy having bubbles blown into their gills, apparently, and this one liked a cuddle too. In one of the loveliest and most improbable moments we’re likely to see on telly all week, Marcus swam with the fish in his arms, as though they were rehearsing an undersea routine for Strictly.

Thanks to advances in forensic fish criminology, experts were able to match up the teeth mark on Friday’s forearm and . . . it wasn’t el Diablo! There must be another submarine psychopath on the loose.

In fact, our naughty nurse shark has settled down since moving into his new tank, with some female company to distract him. Turns out he’s a lover not a biter.

The more dangerous and depressing lawless behaviour in Plymouth was flaunted in the open on Drugs Map Of Britain (BBC3), as a brazen dealer drove around the city delivering bags of a white powder called ‘ket’ — the horse tranquiliser ketamine.

he showed the cameras how he was able to smuggle £1,000-worth of the stuff past security guards at a music festival, by concealing it in a lager can.

At the other end of the country, a teenager called Sky gave the film crew a tour, with enid Blytonish jolliness, of the best places in Fife to get wasted. ‘You can get half a gram here quicker than you could get a taxi,’ she assured them.

This short but excellent series should be compulsory viewing for anyone who wants to understand the state we’re in. One young man, twitching for his next ketamine hit, was counting the minutes till a Disability Living Allowance payout: ‘That’s a free session right there.’