UPDATED 07/05/12 – 3:19 p.m.

CHICAGO (CBS) — The Shedd Aquarium was evacuated Thursday afternoon when the electricity went out to much of the building.

The aquarium’s original galleries lost power at about 12:30 p.m., prompting officials to evacuate the building as a safety precaution, according to a news release from the Shedd.

Life-support systems for the animals at the aquarium were running on backup generators.

Spokeswoman Andrea Smalec said “Things are fine now,” when it comes to the fish and other marine life that reside at the aquarium, which depend on electrical power for the circulation of air in their tanks.

Shedd officials said it appeared the outage was the result of “an internal power issue.”

Many Shedd patrons were confused as they tweeted about the evacuation. At first, some complained that they weren’t told why they were being evacuated.

But upon finding out, many tweeted their disappointment.

“They had a power outage at the Shedd Aquarium, ruined my day,” tweeted Jamal, who identifies himself as a college sophomore.

“Are u offering any makeup days for us being evacuated due to power outage today?” Shelley Hart tweeted to the aquarium. “Tweet if u reopen today, please!”

By about 1:50 p.m., the power at the aquarium was starting to come back on. But the aquarium will not reopen on Thursday. Officials planned to reopen the building on Friday.

The guests who were evacuated can use the tickets from Thursday for another visit, or get a VIP ticket that doesn’t expire.

Representatives of the nearby Adler Planetarium and Field Museum of Natural History say the museums both still have power.