Four years later, nearly 2,000 Japanese koi carp swarm around the 30-metre pool where the locals once bathed. A chorus of bird song rises from the former changing cubicles - now a row of aviaries - that line the pool's perimeter.
The baths, a stunning example of late Victorian architecture, were closed by the city council and lay empty for six years, prey to neglect and vandalism. Every window was smashed and the two swimming pools were full of rubbish. When Mr Allen finally forced open the front door, he found grass growing knee- deep in the entrance hall.
But a number of features from the original 1897 building had survived. In the foyer, he noticed the fish motif in the striking green and cream tiles and the stained glass windows; the seeds of his vision of transforming the place into a gigantic pet shop had been planted.
With the help of a pounds 250,000 loan, the baths in the Tuebrook area of the city have been restored to their former magnificence and opened as the Lister Drive Fisheries and Pet Centre about two months ago. The success of the venture has surprised many people, including Mr Allen. 'Everyone thought I was wasting my time and money. I still wonder about it myself some days,' he said. Mr Allen, formerly in the property business, scratched his head when asked how he dreamt up the idea. 'We were gazumped with the trout farm. So we ended up here.'
As well as 100 varieties of tropical and cool-water fish - housed in the second, 25-metre pool and a solarium built just before the baths closed - the centre offers a veritable Noah's Ark of animal life.
Turtles cavort in what was once a shower basin; chinchillas back-somersault at breakneck speed around their cages in the former bathroom; fire-bellied toads crouch at the bottom of converted lockers. In the aviaries, multi- hued lovebirds croon.
But the koi carp, which can grow to up to 3ft in length, are the centrepiece. Highly prized in Japan, they apparently attract a keen following in Liverpool too, despite their price tag. A 2ft specimen was recently bought for pounds 200 by a local solicitor.
'These people are fanatics,' Mr Allen said. 'They have to be to pay that sort of money.' Most customers, however, are content with smaller fry. 'I make more money from goldfish,' Mr Allen said.