It's the second alert for the canal this month and it's causing added concerns for residents and their pets.
In the back of his home along the C-51 canal, James Sylvester is now worried about his dogs and the water.
"There are quite a few people on Facebook, on the chat of the Lake Clarke Shores, that are really upset about it," Sylvester said.
Residents are upset over the death of Bella, an 8-year-old dog whose owner said died from ingesting blue-green algae in the water.
Pamela McAfee, the dog's owner, said testing on Bella's vomit showed how dangerous the water was, at least when Bella was sick in late May.
"May 24, Bella was sick. May 25, Bella was dead, and not only did she die, [but it was also] a horrible death," McAfee said.
Signs warning of an algae alert are posted at Spillway Park where the C-51 canal leads to the Lake Worth Lagoon.
"The water, the look of it was horrible. It was green. It was blue. It was nasty," McAfee said.
Dr. Malcolm McFarland of FAU Harbor Branch said the toxic algae can come from runoff or Lake Okeechobee water. He said testing cannot offer a complete picture.
"You can get a very low reading in one area and then just a 100 yards away you can get something that was a hundred times more concentrated than that," McFarland said.
He suspects that may have happened with Bella, and pets and people need to now be careful around the water.
"We have neighbors that have kids, that have dogs, that have boats and enjoy the waterway on the weekends, but it's no longer safe," McAfee said.
The South Florida Water Management District has created an algal bloom dashboard where you can report blooms and view the results of algae samples.
This story was originally published by Matt Sczesny at WPTV.