From calves to Koi: the scientific path

Koen’s professional journey began in medical microbiology. “I wanted to work in a hospital, helping people,” he says. But his fascination with parasites led him on a different track. He ended up at a pharmaceutical company where he worked for more than 20 years developing vaccines for animals. His pride and joy: an effective vaccine against Cryptosporidium in calves.

“That trajectory took fifteen years. I’ve been involved from the time the protein was identified and patented it, helped develop the vaccine and oversaw the market launch. It’s in my DNA to want to understand and improve complex biology.”

This systematic, science-based approach is now the basis for his work in the Koi world.

The pond as an ecosystem

Koen approaches the pond as a living ecosystem. “When you install a filter, you’re not installing a machine, you’re installing a biological community. And it only works well if you understand it.”

His search began out of wonder as well as frustration with poor explanations in the hobby. “On forums I got questions, but also many half-answers. I wanted to know the real deal. So I started to investigate. Sometimes literally, all the way to the university.”

That resulted in his first own master class on filter biology – a six-hour session that established his name among serious hobbyists that we may also see appear under the banner of Koi Academy if it’s up to Koen.

Those stories about square feet of attachment area per liter of filter media … those make me red-faced

A story full of questions, insights and nuance

The interview reads like a voyage of discovery through the hidden worlds underwater; from bacteria in filters to virus biology, from pH influences to persistent hobby fables, and gives a good impression of what animates Koen today;

About adhesion surface in filter media

“Those stories about square feet of attachment area per liter of filter media … those make me red-faced,” he says. “Not because those materials are bad, on the contrary, but because it’s often such an empty claim. The real question is: What is actually happening biologically in that volume? What bacteria live there? How fast is the water flowing? How much oxygen is there?”

pH, oxygen and bacterial dominance

We talk about bacterial strains, about how a pH of 7.2 produces a completely different microflora than a pH of 8.0. About how oxygen shifts the balance between autotrophic and heterotrophic bacteria, and what that means for your pond. “Most hobbyists have no idea how much influence parameters like pH and oxygen have on which bacteria become dominant. And that’s crucial if you understand why your filter does or doesn’t or does better or worse.”

CO₂-outwash in moving bed filters

“That CO₂ loss causes nitrite spikes? I didn’t believe any of it and started testing it against data. It’s much more nuanced.” The idea that moving bed filters fail due to CO₂ efflux also comes up. Koen smiles, “I hear that theory a lot, but I didn’t believe it. So I started looking into it. Not with opinions, but with data. I talked to scientists, analyzed literature, and only then did it fall into place. It’s more nuanced than people think.”

Disease as symptom, not cause

The conversation shifts to illnesses. Koen makes an important point: “What people often think of as ‘the disease’ is usually not the primary problem. A bacterial infection is often a consequence. Poor water quality, stress, a malfunctioning filter; these are the real causes. And that’s where you need to focus.”

KHV vaccine: critical look at genetic stability

Then another heavy topic slides in that I had to broach given his experiences in vaccine development: the Israeli KHV vaccine. Koen is cautious. “If it is true that only three base pairs are genetically modified, then that is very little. Viruses are smart. They adapt themselves. And a live weakened virus that can recover … you have to be careful with that.”

These topics show not only Koen’s brainpower, but also his ability to make science accessible. Many of these will soon be further elaborated with him in in-depth articles on KoiQuestion. Koen was also the brains behind the app the Koi calculator by the way; another example where he tries to make science accessible to the hobbyist.

Also read: Koi Calculator under the microscope

Education as a mission

• Filter biology under variable conditions

• Diagnostic system knowledge

• Preventive fish health through water management

• Applied bacteriology and virology