We’re invisible to the eye and therefore completely uninteresting to most pond owners. We are the large community of bacteria that live in the pond and make sure that it functions properly. You only think of us when something goes wrong. But we are proud, we don't hold it against our pond master.
We are quite undemanding and can survive anywhere: In the substrate, on stones and wood, but especially in the filter. Because that's where we find our best living space, offering room for so many of us. There’s often more of us in the filter than in the pond itself. We’re not too choosy where we live: a wide variety of plastics, ceramics and sintered glass materials are fine. But it is odd that many manufacturers of these materials talk about them having small channels in which we can work particularly effectively. They don't seem to know that after a short time biofilms close these pores and channels stopping us from getting in or out. A filter material that is too fine is just not sensible.
What is more important is that this filter material is designed to clean itself, as we are quite selfish. We simply grow on existing colleagues and form thicker and thicker layers. The lowest ones die off and the whole bacterial lawn cracks. If the filter material is too fine, the lawns will get stuck and clog the filter. Whereas on coarser material everything is washed away, making space for new colleagues.
We are also happy about pre-filtered water, because pond water with a high mechanical dirt content overtaxes us. Once the coarse dirt is removed, we can work better. Then we are able to process nitrogen compounds such as ammonium (NH4) and nitrite (NO2) into non-toxic nitrate (NO3) without problems. We do this work without being asked and gladly, but only if there is enough oxygen. And that is exactly where the problem often lies! We don’t like the water flowing too slowly through the filter, and with larger filters we prefer the water to be aerated. The more oxygen we have, the better we work. If we’re going to work nonstop, we need the best possible working conditions! Some of our masters are energy savers and switch off the filter at night. Great idea! How are we supposed to survive the night? Without oxygen we have to change our metabolism or die. The sludge full of our dead colleagues is then flushed back into the pond in the morning when the filter is turned on. How daft can you get?
Talking about mass murder: Some pond owners seem to think that salt is good for the pond. Let's not go into whether salt has any benefits, but the fact is: we bacteria can't cope at all with this sudden increase in salt content and are collectively sent to meet our maker. And it takes a hell of a long time to form a stable bacteria colony again!
We reproduce in a somewhat peculiar way. There are only as many of us as there is food for us (i.e. nitrogenous substances such as protein, ammonium, nitrite and nitrate). If the water load suddenly increases, we don't react immediately, there’s a delay. But because of this slow start-up phase (lag phase), the toxic nitrogen substances can rise to levels that can be very dangerous for the fish. And when we finally get going, we multiply exponentially, i.e. very quickly. Again, this means that it is important to create the right working conditions for us. Oxygen is essential.
But not for all of us! Some of us are really versatile: if oxygen is present, they break down proteins via ammonium and nitrite to nitrate. However, if the oxygen content drops, as can be the case, say, in thick substrate or in very slow-running filters, they adapt their metabolism and fetch the missing oxygen from the nitrate (NO3). In nitrate, three oxygen atoms are bound to one nitrogen atom. We bacteria then crack oxygen for oxygen out of the nitrate molecule. Although toxic nitrite (NO2) is formed for a short time, it is quickly processed into harmless nitrogen gas (N2). Nitrogen only leaves the water as nitrogen gas. Although nitrate is harmless to fish and is a nutrient for plants, it also forms the nutritional basis for algae together with phosphates.
Some pond owners connect UV-C water clarifiers IN FRONT OF the pond filter. This is okay as long as we have already settled in the pond filter, because otherwise the UV-C radiation is deadly for us bacteria. If the filter is new or has been completely cleaned, an upstream UV-C water clarifier will prevent bacterial colonisation. So it makes sense to switch the UV-C water clarifier for a month BEHIND and only later put it IN FRONT OF the filter.
Finally, I would like to introduce some colleagues of mine to you. We call them the heterotrophs and they can do something very special: They can convert organic matter such as leaves and dying algae in the filter and at the bottom of the pond into ammonium and ammonia. Without our heterotrophs, we would not be able to work at all, as we can only start with further degradation when ammonium is present. Unfortunately, our heterotrophic colleagues are often prevented from working - especially at the bottom of the pond. This is because they need a lot of oxygen to work, and oxygen is exactly what is missing there. The more they work, the lower the oxygen content becomes. Some pond owners are smart enough to add heterotrophic bacteria and active oxygen as a secondary component to the pond. These then work together to break down the pond sludge. In our filter, this problem does not occur so much because the water flowing through it usually provides enough oxygen.
Unemployment is a rare problem. Which is good, because if we have nothing to do, we die. Only as many of us survive as there is work for us to do, i.e. food in the form of proteins, ammonium, nitrite and nitrate. If the amount of work increases, we reproduce more strongly again, but with the delay effect I explained at the beginning.
We bacteria have simple needs. Most of us (aerobic nitrifiers) only need a lot of fresh air (oxygen) and some of us (anaerobic denitrifiers) only work when oxygen is lacking. In new ponds and in cleaned filters, pond owners should definitely add a few million of us, otherwise we take too long to reproduce. Hardly any of us swim around freely in the water. If we do we’re more likely to be those renegade colleagues who can cause diseases. We workers always need a solid base (substrate) on which to live. We don't particularly like floating free!