When the former German army officer first arrived on the tropical island four years ago he thought he had found his little piece of paradise among the gently swaying palms, white beaches and crystal clear waters that surround the island.
But not all was as it appeared in the popular diving resort.
"When I came here four years ago dynamite fishing was rampant," he tells Agence France Presse. "And as you know diving and dynamite fishing do not belong together. So I started to fight back."
The tall and athletic Berliner, now 45, is an unlikely guardian of the coral colonies of the Southeast Asian archipelago. Some of the most spectacular formations can be found off Panglao and the nearby islands of Balicasag and Pamilacan.
A dive instructor by day, Horn doubles as a board member of the Bohol Islands Marine Triangle program, funded by the UN Development Program to conserve the marine life of Bohol Sea, a major highway for the transoceanic traffic of sea creatures from giant whales to tiny krill.
For his efforts, Horn has won an honorary commission into the Philippines Coast Guard.
"In the beginning nobody cared. Then I took over to care for the reefs and now you can see great areas of improvement. It’s really wonderful to see how nature is rebuilding," Horn says.
Compared to those of many other countries, "the diversity of the Philippine coral reefs is still quite in the upper level," he added.
Last year, a French-led research team identified some 1,200 different species of crabs and shrimps and around 6,000 mollusk species in the waters off Panglao, many of them not previously known to science.
But pressure from fishing fleets and marine tourism, funneled into Panglao from major tourist gateways like nearby Cebu City, have put the vast underwater gardens in jeopardy.
The environment department says the destruction of the reefs, breeding grounds and sanctuaries for many of the creatures of the sea, went into overdrive in the 1970s when fishermen tossed dynamite or poison into the water to stun or blind the fish.
Some of the damage was not even deliberate.
"There are types of corals that grow about 10 centimeters in one year. So for example, you throw an anchor at a coral garden and you destroyed two square meters. It takes five to 10 years to get it to grow back," Horn says.
From the start, conservation was an alien concept to many residents, who have fished the sea for generations.
"But the perception has changed," he says, a point driven home by the reality of declining catches.
"Look at them today, the marginal fishermen. They come back with a handful of fish and they can’t even feed their families," he says.
The government has restricted fishing in shallow waters to small fishermen, set up community-based marine sanctuary projects, and encouraged marine tourism including scuba diving and whale watching.
Some 15,000 scuba divers visit Panglao and the nearby islands every year, drawn by 18 established dive sites in the area.
The son of a marine biologist who instilled in his son an abiding fascination with the sea, Horn says cultural factors also play a big influence in marine conservation.
"I found out that the Philippines has great laws, rules and regulations and ordinances but they really lack implementation," he says.
One reason, he says is that these laws are foreign to many residents. "You are dealing here with people who spent one or sometimes three years in elementary school, and sometimes they don’t even know how to write properly. So how do you make them understand that these ordinances are so important?"
Even some of the rich and presumably better educated tourists disappoint him because they take trophies or souvenirs from the dive sites.
"Our base for an effective tourism industry is the coral reefs. So if we don’t have intact coral reefs, the resorts can’t (attract tourists) and will have to close down," he says.
Within the project site, only traditional means of fishing are now allowed. Permanent buoys are planned so vessels would stop using destructive anchors in the protected site.
The Bohol Islands Marine Triangle board is supervising a zoning survey to delineate which areas to declare as sanctuaries, fishing areas, tourism sections or navigational corridors.
Also planned are a user-fee scheme under which every scuba diver would be charged P50, and boats that damage corals by indiscriminately dropping anchors would be fined. Proceeds would pay for fish wardens to guard the sanctuaries and support communities involved in the project.
The Panglao municipal council also passed an ordinance requiring new restaurants or hotels to put up proper waste disposal systems.
Horn says Panglao is learning its lessons from Boracay, another world-famous island resort in Aklan that hit the headlines for all the wrong reasons a few years back due to E. coli bacteria infection of the water.
Horn, who runs his dive shop with his Japanese girlfriend, says he loves the Philippines and hopes to "settle down" in Panglao eventually.
"It’s more than the diving actually. I like the culture. The Filipino culture is quite easygoing. The climate is quite nice. The people are normally nice and friendly and cheerful."