In this interview, Dr. Gerald Bassleer outlines a practical, prevention‑first approach to aquarium health. He emphasizes three core measures: vigilant observation of fish behavior, regular water testing, and feeding functional, probiotic‑rich fish food. Stress reduction is highlighted as the overarching goal, with CBD‑infused feed suggested only for newly introduced fish to ease acclimation stress. Bassleer critiques commercial “bacteria‑in‑a‑bottle” products, sharing results from a study with Aquarium Münster that showed bacterial viability of up to six weeks without food or oxygen, and describing the Bacto Sprint fluid system that rapidly reactivates freeze‑dried cultures. He advocates inoculating new tanks with media from an established, mature filter rather than relying on bottled starters. The size and capacity of the biological filter are presented as the primary determinant of disease risk; larger, well‑colonized filters act like a strong immune system, while undersized filters lead to ammonia spikes and disease. Bassleer discusses filter media options, favoring coarse sponge filters for ease of cleaning and bacterial retention, and stresses that media cleaning should be done in tank water at similar temperature to avoid shocking beneficial bacteria. He also debunks the myth that over‑filtering is harmful, explaining that bacteria grow only in proportion to available waste. Throughout, he frames disease prevention as addressing root causes—water quality, nutrition, and bio‑filter health—rather than treating symptoms.
Chapters
From your book, could you share one of your favorite preventative measures against fish diseases? I've got a feeling I know what you might—
You mean preventative measures?
Yeah,
your favorite one. The most preventative measure is, like I said in the beginning of our talk, watching your fish. Yeah. Number 2, water testing. Number 3, buy a good fish food. With a functional fish food that helps the fish with probiotics, prebiotics, functional food to fight off or to prevent diseases, things like that. Those are the 3 basic things. And avoiding— this is all about avoiding stress. You know, this is the— that's the main thing. Yeah.
I had a feeling that might be part of your answer there.
You're talking— I would give you a magic bullet or what?
Fish food. Sorry, Cam, just before we go into the next question, fish food and avoiding stress. And was that the reason for adding CBD into one of your fish foods?
No, no, no. CBD is only meant to particularly when fish arrive in an aquarium. New fish. There's a lot of stress.
Okay.
That slows down. Well, put the fish in a little good happy mood. How can I say it? It's it's it particularly when new fish are introduced, the shoes leak some aggressivity, a risk for. Or when when you move fish, you move your to another new apartment, or you move your aquarium. Those things might help the fish to avoid. Well, if you take CBD, you know what it means in your life. Yeah. Life is happy and good. So so I know this about the basic idea of CBD to to to avoid well to lower the lower stress situations.
Okay.
Yeah,
interesting.
Uh, another comment from the floor. Can you comment about tank cycling starters or bacteria in a bottle, and is there any good ones out there?
Well, uh, there are some good ones out there. The best starter for a filter everybody has at home available. It's your filter. Your filter— if you start up a new aquarium, take a part out of your existing filter, some ceramic balls or some part of the sponge, you cut it off, and there are millions of bacteria in there, and that you inoculate in the new aquarium. You have bacteria in a bottle, out of the bottle. Those in the bottle, well, uh, to be honest, you know, I've been part of a study with Aquarium Münster. How long does bacteria live in a bottle? Maximum 6 weeks.
Wow.
6 weeks without oxygen. They die within 6 weeks without food. So if, if you have your aquarium you shut down the biological filtration or you put it in a bucket, you soak it in water, if you put air in there or not air for a couple of days, it doesn't kill the bacteria. It stays alive for 6 weeks, at least a couple of weeks. The longer you wait, the less bacteria because there's no food, there is no oxygen. So we, we did a study with the University of Switzerland To develop a bacteria in a bottle and a fluid, and that's Bacto Sprint by Aquari Münster, which I like as one of the better ones that I know because it's a fluid in a bottle, and in the top it is a cap with bacteria freeze-dried. You screw open the bottle, the bacteria fall into the fluid, which is a food for the bacteria. And within a couple of minutes, you see the water bubbling, changing. The bacteria come alive. You even can smell it.
Wow.
So you have live bacteria within minutes you administer to the aquarium. Still, they need to grow, to settle down. Takes time. But this is a, a way of making a live culture. The best bacteria are the ones in your aquarium. So I started my 1,000 aquariums in my fish house with one aquarium, no bottles, no products, with one biological filter which was working. And I inoculated that, embedded it in all my filters piece by piece.
Amazing.
I mean,
it's interesting because, Cam, you've done this study not that long ago about Cycling aquariums. And ultimately, it was taking filter media from an existing mature tank that came out as the most reliable method, wasn't it?
Yeah, absolutely.
Yeah, yeah. For you, Cameron, if you want to have a hobbyist who wants to have a public aquarium in his restaurant tomorrow, it has to work tomorrow. Tomorrow, all the fish in there, 1,000-liter Big business.
Need it yesterday.
What do you do with the filter to make it ready? You separate it.
Ready to go. Yep.
Your filter from your fish house, you give them a part of your sponges and you will be happy. No problems. As long as you doesn't overcrowd the aquarium, of course.
And then
bacteria attack. That's the magic. It's no magic around it. You know bacteria. are in our, in our aquaria. Everybody has the bottle of bacteria at home, and if he buys a second aquarium, use that bacteria. Some people think, well, I might transport a disease. You know, there is hardly any disease surviving in the biological filter. They're eaten by the bacteria. They cannot survive, except some rare incidents of eggs of worms who are encapsulated. But usually they don't survive for a long time. They need a host, or even white spot disease—it's eaten by the bacteria. There's hardly any parasites surviving for a long time in in the biological filter. It's not their way of surviving.
Can you talk to that a little bit more? I wasn't aware that that worked that way. I was always my only main concerns about transferring one. filter to another, for example, is potential disease outbreak and snails that could be moving from one tank to another, or worms and that kind of stuff. So I wasn't aware that the bacteria within the biological media would consume that. So can you talk to that a little bit, please?
Well, because the biological filter contains so much bacteria, this is not the habitat for the parasite. White spot needs the mucus of the fish, needs the gills of the fish. The parasites need— the bacteria needs the host, and the sponge biological filter is not the host. It's a temporary way of surviving maybe, but they are eaten by the bacteria and the biological filter. They have no way— it's not their host. They need Maybe temporarily they can be sucked in, you know, by as a parasite, or like particularly the eggs of gill flukes. Yes, the eggs of gill flukes can appear because they are encapsulated and that bacteria cannot— how can I say— eaten that larvae or that worm larvae. It's the same for eggs of Capillaria. nematodes. Those are the few exceptions, you know, that, that's why the medications recommend to treat within 1 week again, because those larvae or those eggs can produce larvae, which you have to kill again with the new retreatment against the worm, against gill flukes. Mm-hmm. Particularly gill flukes. That's, that's a typical case. You have to retreat within 1 week or 10 days. To kill off the larvae which might appear. But after three four weeks they die. They have no host. They will maybe pop out of their their egg or their shell, but they will be eaten by the bacteria. There's no this there's no host tissue there. It's not their final host. So there is very little risk. I always say sometimes people think oh I must disinfect my filter. Because a lot of parasites or bacteria might be in there, and that can cause a reinfection. This is not true. This is you destroy your biological filter. You good bacteria—they are the ones who controlling the healthy biological filter, and they will make sure it's cleaned out as long as you have a good biological filtration. That is good enough, big enough to handle it all. The smaller your biological filter, the less opportunity it has. It's the same like a sportsman. A sportsman has a big heart, well-developed, well-trained. An aquarium must have a good developed biological filter, big enough to handle the waste, big enough to handle the volume of fish. Yeah. When it's too small, you have issues, you have problems— ammonia, nitrite, anything. I always tell the people, the best investment in your aquarium is a big biological filter. The bigger the better, the bigger the better. Of course, there's a limit, but the bigger the better, your biological filter. And I tell you, in my fish house I had 1,000 aquaria, and each aquarium, one-third— hear me, 30% of my aquarium was biological filter. 30% of each aquarium was biological filtration. My prevention against diseases, my prevention against a lot of water changes, my prevention against a lot of work. I could do to make money.
So that, that's brought up 2 things. The more biological capability your filtration has, does that mean the lesser chance of having fish disease?
Yes, certainly, certainly, certainly. Of course, there are other factors that play a role, you know, nutrition and temperature, But anyway, the filter, the biological filter does more than eliminating waste. It controls the health of the aquarium. Yes.
And the next thing that's brought up with me is whether it be a myth or a common belief that your filtration can only handle as much as your fish can produce. So one guppy with 10 kilos of biological filtration would then seem to be an excessive waste. Would that be correct in saying that, or will it just continue to colonize over everything and continue to grow with what it is? Or does it need a proper element of being fed to continue to, to feed through?
Yeah, thanks for bringing it up, Cameron, you know, because The biological filter, the the content, the bacteria only grow and colonize depending on the amount of food they get.
Yeah.
So if you have only two guppies, you will have a little amount of bacteria.
Much like snails.
Hey? Yeah.
Very similar to snails in your gravel.
Yeah. Yeah. The the thing is, you know, and then people have two two guppies, you know. And have a biological filter running for four or five weeks, and they're putting in then five angelfish, twenty Corydoras, thirty cardinals. Whoa! I have ammonia nitrite. Yes, your biological filter does not know how to handle the waste of those sixty, seventy extra fish. This has to be teached. That has to grow. Yes, John has to be balanced. So you have to give proper time. So I always tell the people, slowly adding fish, let the bacteria grow with the system. Yeah,
so extra biological allows you to have more fish because there's more to colonize. However, there— the, the belief and the, the thought that you can over-filter your tank still is applicable because 1 guppy and 10 kilos of bio is not going to colonize your bio correctly.
Yeah.
So that will make sense. Yeah.
I've seen an experiment in university in Holland, in Wageningen. Wageningen is one of the biggest universities on aquaculture. And they did a study that put 1,000 trouts, believe it or not, in 100 liters of water with a filter of 1,000 liters. And they did well.
Wow.
They did well because the filter determined how much fish you can keep, not the water volume. Of course, on the long run, 1,000 fish in 100 liters of water— well, the trout didn't mind because they like to live in schools, you know,
and
they didn't mind. But anyway, it's to show There was no stress from water quality. Yeah, the water quality was controlled.
Yeah, all seen the videos from overseas and different parts of the world where heaps of fish are chucked into a small tank, but it's so incredibly strongly filtered. Yeah, there's no issue with it. It looks quite ghastly, but water parameters under control because there's so much media in there to deal with all the waste that comes through.
Yeah, yeah, correct. The biological filter has, has to know, you know, the, the filter controls the capacity of your aquarium. How many fish you can keep? That's the filter. The smaller your biological filter, the smaller amount of fish. That's it. Accept it. If you have a small biological filter, you want to save, and you buy a little can, a canister, okay, be happy with a few fish. But if you have a lot of more fish keeping in there, you're gonna have permanently problems, diseases, and you wonder why. Yes, because it's like a sportsman. A marathon runner cannot run when his heart is not well equipped to run the marathon. That's the same for the aquarium.
Okay, that kind of leads on to a question that we've got from Helen in the chat about filter media. And she's asking what the best or most effective form of biological filter media actually is, in your opinion.
Okay, well, there is never the best form. Uh, any, any form can collect bacteria. Mhm. The more, uh, substrate there is where the bacteria can attach, like, uh, plastic balls, you know, you have all, all those different materials, you know. Uh, I always worked with, uh, sponge filters.
Mhm.
Not dense but very coarse sponge filters. And we did a study on filters. As long as the media can collect a good amount of bacteria, people using wool, you know, just the sponge wool just collects a lot of bacteria. Yeah. You think it's a dirt filter. It has a lot of bacteria, but the sponge you can easily clean. You can maintain, you can wash it out. In tank water and put it back so the waste gets out. So it all depends also on the maintenance of that media, how good it is. You know, I, I like sponges, coarse sponges, because you can easily handle them, take them out and clean them. That's easy. And the way to clean it is on a regular basis, depending on the amount of fish, the amount of waste. My aquarium, which I have here, I only do once a year a filter maintenance. Why? A big aquarium and few fish.
Hmm.
And only feed twice a day, a little bit, enough that they have food. A lot of people give too much food, so they have a lot of waste, a lot of problems. But anyway, the maintenance of the filter is important that you wash it in tank water and In a bucket, not in cold tap water, but tap water has chlorine. Wash it in tap water, just get the biggest dirt out. You don't get the bacteria out. It's the dirt. I put it back in every couple of times a year is enough, depending on your system. Mm-hmm. So it's the media, how you maintain it and, and, and, and how it can collect bacteria. And there are so many good materials available in the industry, you know, sponges and balls. Bio Balls, many good things. Yes, yes, it's— I think the most important thing is how you take care of that medium in the long run when you have it. That's, that's, that's the most important part.
There's— you've just mentioned then cleaning out your, your filter media in, uh, tank water. There's— I believe there's been a study that's been done Cleaning your filter media out in tap water and the biological number of bacteria, I'm not sure how they tested it, was seen to actually not be affected all that much or very little. Have you done any tests on that? Have you seen any results on that sort of thing?
Yeah, what happens when you wash your sponge, for example, in cold tap water, you stress and kill the bacteria because the bacteria used to live in tropical water. But you have a tropical aquarium. And there is sometimes tap water has chlorine, you kill the bacteria. So, uh, you can wash it in the tap water as long as the tap water is the same as the tank water. Yes, no problem, Cameron, that you, that you do that as long as the tap water is the same as the aquarium water, as long as the temperature, because you can shock the bacteria. When you wash it in temperature of ten degrees, particularly Europe. Hey, Scotland! Hey, John, how cold is the tap water?
Far too cold to put straight in an aquarium anyway.
Two degrees in the middle of summer.
I cannot clean the sponge in the tap water of Scotland already. Oh no!
Forget it. It's got perfect pH for fish, but the cooling is just not great.
Yeah, yeah.
So that would be saying that the back— if you have a goldfish tank in cold water and a tropical tank, couldn't flip them over, the sponges over, for example, and then bacteria be okay? Although it could be the same similar strain, the shock of water temperatures will be enough to kill them off.
It can shock the bacteria, of course. Yes, it can shock the bacteria. They're used like a fish to live in that water. Yes. Yeah. Yes.
Cool.
See, we talk about different things than diseases for the fish doctor.
Yeah.
But it has all to do with diseases. This is the causes, which is, I think, more important than the diseases. I like to prevent the diseases by talking about these issues. Thank you. Thank you.
It's healthy ecosystems and what's going to be best for the fish that we're keeping in glass boxes at the end of the day.
