Work to install the centrepiece of Bristol's new £4-million aquarium is under way.

The aquarium's underwater walk-through tunnel, allowing visitors to walk along the seabed of a coral reef surrounded by tropical fish and sharks, was delivered yesterday in sections from a factory in Weymouth.

More than 7,000 fish from 250 species will be on display across 40 different attractions.

The aquarium, on the site of the former Wildwalk attraction on Harbourside, is set to open for the October school half-term.

It will shortly begin recruiting around 40 staff to man the attraction.

Four tunnel sections, as well as a 4.5m by 2.5m window, are being installed in the aquarium's new 200,000 litre coral reef display.

The coral display will include zebra sharks, stingrays and pufferfish.

A second 3m by 2m window is also being delivered for the new aquarium's 260,000 litre "shipwreck display".

This will include cod, bass, lobsters and crabs.

Each tank will be made up of filtered sea water brought by tanker under licence from the South Coast.

Each tank takes around three days to fill from a reservoir on the premises.

Ultra-violet technology is used to filter the water.

Since glass is not strong enough to withstand the pressure, the sections have been manufactured from lightweight, transparent acrylic plastic.

Despite being 10 centimetres thick, it has been calculated that each panel is capable of sustaining a water pressure of 2,000 tonnes.

Blue Reef manager Alex Page said: "The arrival of the tunnel and other window sections is a key milestone in the development of the new aquarium.

"Once secured in place they will allow us to fill the giant ocean tanks for the first time in readiness for the arrival of thousands of exotic tropical fish – including black tip reef sharks – over the coming weeks."

Each section of the tunnel begins life as a flat section of acrylic which is placed over a curved mould and then "cooked" in a giant oven. The acrylic is then handfinished and tailor-made to fit the exact measurements of the individual tunnel.

It takes around three days for this process to be completed.

As well as four sections of tunnel and the "tropical" viewing window, the new display will also feature a giant "fish-eye" viewing window.

The window, which measures close to two metres square and is 50cm deep, will give visitors a 360-degree magnified view of the tank and its occupants.

The Wildwalk/Imax site, next to @Bristol, closed in April 2007 after operators revealed it was losing £1.5m a year.

The Imax will now reopen, showing 3D nature films, alongside the aquarium in October.