Biological filtration is typically explained as the collaboration of two groups of microorganisms. However, some bacteria can manage both phases on their own — and research has found them in all 38 studied freshwater filters.
Many aquarists are familiar with the basic description of nitrification: ammonia is converted into nitrites and then into nitrates. The first transformation is ensured by one group of microorganisms, while the second is handled by another. The chemical sequence holds true, but the transformation can also be carried out by just one group of microorganisms. Some representatives of the genus Nitrospira can indeed perform the entire process in a single cell. They are referred to as comammox - microorganisms capable of complete ammonia oxidation. They possess the necessary enzymatic equipment for both steps and gain energy from the conversion of ammonia through nitrites to nitrates. This ability does not automatically apply to all bacteria of the genus Nitrospira.
A study published in 2024 examined filters from freshwater aquariums in homes and stores in Canada. Genetic markers of comammox bacteria were found in all 38 samples. In 30 of them, their monitored genes predominated over the corresponding genes of other studied groups that oxidize ammonia.
What does this imply for setting up an aquarium? Primarily, that even bacteria capable of handling both phases of transformation do not immediately indicate a functional filter. In another study, scientists monitored three newly established home aquariums for twelve weeks. In two of them, ammonia and nitrite levels dropped below detection limits by the third week of monitoring, while in the third, it took until the eighth week. Comammox bacteria gradually colonized all three filters.
The practical conclusion for aquarists is as follows - when cycling an aquarium, monitor not only ammonia and ammonium ions but also nitrites. A decrease in the first group of substances does not necessarily indicate that the second step is safely occurring. In fact, in the two monitored tanks, ammonia levels were already undetectable, while nitrites were just reaching their peak. Making decisions based on repeated measurements makes more sense than relying on a fixed number of days for cycling.
Sources used
Van Kessel et al. (2015): Complete nitrification by a single microorganism. Nature.
McKnight and Neufeld (2024): Comammox Nitrospira among dominant ammonia oxidizers within aquarium biofilter microbial communities. Applied and Environmental Microbiology.
McKnight et al. (2025): Microbial community succession of home aquarium biofilters associated with early establishment of comammox Nitrospira. ISME Communications.
Spolek plzeňských akvaristů IRIS zve všechny příznivce akvaristiky na podzimní akvaristický trh.
31.října 2026 (sobota), od 9.00 - 11.30 h.
The South Bohemian Aquarists Association invites all enthusiasts of aquaristics, terraristics, and nature lovers to Aquatrade of aquarium fish, plants, accessories, and wildlife protection.
Come and shop from experienced breeders, replenish your supplies, or just chat with other enthusiasts in the field!
- Date: Saturday, October 17, 2026
- Location: Třeboň – Secondary Fisheries School, Táboritská 941 (school cafeteria)
- Admission: 50 CZK (children up to 10 years accompanied by parents free)
- Organizer: South Bohemian Aquarists Association (Akvatera Č.B.)
- 07:00 – Entry for sellers
- 09:00 – 12:00 – Aquatrade for the public
The Aquatrade takes place under expert veterinary supervision (MVDr. J. Konrád).
Do you want to offer your breeds, plants, or equipment at the market? Please send your sales applications in advance:
- Email: akvateracb@seznam.cz
- Phone: +420 605 714 634
More information can be found on the organizer's official website: akvatera-cb.aspfree.cz
Dear aquarium friends,
I would like to invite you to our international killifish exhibition, which will take place from Thursday, September 17th to Sunday, September 20th. On Saturday at 2 PM, there will be a lecture on the genus Nothobranchius. Starting Sunday morning, there will be an auction of the exhibited fish, and anyone interested can bid. Over 250 collections of fish from Europe and Asia have been registered for the exhibition. You will be able to see interesting and rare species of killifish, including species that are extinct in the wild, and you will also have the opportunity to learn more about their life, breeding, and reproduction, as well as talk to the breeders of these beautiful and overlooked aquarium fish.
Again, the shrimp are released outside in the yard. Since April, when I introduced about 40 of them into the outdoor tank. Today, around 800 were harvested. Photos from the beginning gradually up to the harvesting and moving to the indoor aquariums.
Spotted Corydoras: An Undemanding and Social Bottom Dweller
The Spotted Corydoras (Corydoras paleatus, more recently Hoplisoma paleatum) is one of the most popular aquarium fish that inhabit the bottom. It is peaceful, social, interesting to observe, and under the right conditions, also relatively easy to care for. Although it is often referred to as an "aquarium cleaner," it requires its own quality food and definitely does not replace regular maintenance of the tank's substrate.
Spotted Corydoras at a Glance
- Czech name: pancéřníček skvrnitý
- Traditional scientific name: Corydoras paleatus
- Newer scientific classification: Hoplisoma paleatum
- Origin: southeastern part of South America
- Size: approximately 6–7 cm
- Behavior: peaceful, social
- Recommended number: at least 5–6 individuals
- Place in the aquarium: primarily the bottom
- Diet: omnivore
- Suitable substrate: fine sand or smooth fine gravel
- Suitable for beginners: yes
What Does the Spotted Corydoras Look Like?
The Spotted Corydoras is easily recognizable by its grayish to silvery body with irregular dark spots. The coloration of individual fish may vary slightly.
A characteristic feature of corydoras is the two rows of hard bony plates along the sides of their body. These provide them with protection and gave the entire group the Czech name "pancéřníčci" (armored fish).
Around their mouths, they have sensitive whiskers that they use to search the substrate for food. This is why it is important to choose a suitable bottom without sharp edges.
Is the Spotted Corydoras Suitable for Beginners?
Yes. The Spotted Corydoras is generally considered a fairly hardy fish that is well-suited for a first community aquarium.
However, this does not mean that it can live in any conditions. Like other fish, it requires stable water quality, appropriate feeding, enough space, and, above all, the company of its own kind.
One of the most common mistakes beginners make is acquiring a single corydoras as a "bottom cleaning fish." Corydoras are social fish and thrive significantly better in groups.
How Many Corydoras Should Be Kept Together?
The Spotted Corydoras should not be kept alone. An ideal group consists of at least 5 to 6 individuals, and more if the aquarium is large enough.
In a group, corydoras are:
- more active,
- less shy,
- more mobile throughout the tank,
- exhibit more natural social behavior.
It is very interesting to observe several corydoras searching the bottom together or resting side by side under a root or among plants.
What Size Aquarium Does the Spotted Corydoras Need?
For a small group of corydoras, an aquarium with a length of at least around 60 cm is suitable.
For these fish, the height of the tank is not the most important factor, but rather a sufficiently large bottom area. They spend most of their time on it.
A suitable aquarium should offer a combination of:
- open areas for searching for food,
- fine substrate,
- plants,
- roots,
- stones without sharp edges,
- quieter places for resting.
Corydoras are well-suited for a classic community aquarium with peaceful species of similar requirements.
What Type of Bottom is Best for Corydoras?
The ideal substrate is fine aquarium sand.
Corydoras bury their mouths and whiskers into the substrate while searching for food. Fine sand allows them to exhibit their natural behavior and minimizes the risk of injury to their sensitive whiskers.
A fine smooth substrate without sharp edges can also be used.
On the other hand, very coarse or sharp gravel is unsuitable.
The cleanliness of the bottom is also important. A long-neglected substrate can contribute to damage or infections of the whiskers, especially in unsuitable conditions.
Is the Corydoras Really an "Aquarium Cleaner"?
Only partially. Corydoras constantly search the bottom and can find small food remnants that other fish have overlooked. This allows them to help utilize some of the food that has fallen to the bottom. However, they are not a substitute for aquarium maintenance.
Corydoras do not eat feces or debris, and they will not do the siphoning for you.
What to Feed the Spotted Corydoras?
The Spotted Corydoras is an omnivore and requires its own regular and varied diet.
The basis can be high-quality:
- sinking pellets,
- tablets for bottom-dwelling fish,
- quality wafers,
- frozen food,
- live food.
As a treat, you can use, for example, brine shrimp, daphnia, or other small animal food suitable for aquarium fish. It cannot be assumed that corydoras will be satisfied with just the leftovers of other fish's food.
In an aquarium with very active fish, it is even advisable to monitor whether a sufficient amount of food actually reaches the bottom.
Why Does the Corydoras Swim to the Surface?
One of the most interesting behaviors of corydoras is their quick trips from the bottom to the surface. The fish swims up, takes in air, and immediately returns back down. This is not necessarily a sign of oxygen deficiency.
Corydoras have the ability to utilize oxygen from atmospheric air as well. The swallowed air passes through their digestive tract, and they can absorb some of the oxygen through a specialized part of their intestine. Occasional trips to the surface are therefore completely normal.
However, if corydoras start surfacing very frequently and their behavior suddenly changes, it is advisable to check:
- water quality,
- temperature,
- filtration,
- water flow,
- oxygenation of the tank.
How Does the Spotted Corydoras Behave?
Corydoras are among very peaceful fish. They spend most of the day searching the bottom. They alternate between actively searching for food and short rests on the substrate, under roots, or among plants.
In a larger group, it is often possible to observe several individuals moving together. Although they are considered bottom-dwelling fish, they are not strictly bound to the bottom. Occasionally, they make quick trips to higher layers of water or even to the surface.
With Which Fish Can Corydoras Be Kept?
The Spotted Corydoras is particularly suitable for a calm community aquarium.
It can work well with, for example:
- smaller tetras,
- rasboras,
- some livebearers,
- calm barbs,
- other peaceful fish of similar size.
It is important to choose species with similar environmental and water temperature requirements.
Inappropriate companions are primarily significantly aggressive or very territorial fish that could attack corydoras or prevent them from accessing food.
The Spotted Corydoras is Not a Catfish
Corydoras are sometimes incorrectly referred to as catfish. However, they belong to a different group of catfish.
The Spotted Corydoras belongs to the family Callichthyidae, while the well-known catfish, such as ancistrus, belong to the family Loricariidae. Although both groups often inhabit the bottom areas of the aquarium, their way of life, body structure, and diet differ.
Corydoras paleatus or Hoplisoma paleatum?
Experienced aquarists primarily know the Spotted Corydoras by the name Corydoras paleatus. However, after a recent taxonomic revision of corydoras, this species has been reclassified into the genus Hoplisoma. Therefore, the current scientific designation is Hoplisoma paleatum.
In aquarium shops, literature, and on the internet, we will still encounter the older designation Corydoras paleatus for a long time. For breeders, of course, nothing changes – it is still the same Spotted Corydoras.
Why Should You Get a Spotted Corydoras?
The Spotted Corydoras offers a combination of traits that have made it one of the classics of freshwater aquaristics.
It is peaceful, active, social, and well-suited for a variety of common community aquariums. While searching for food, it constantly explores the bottom, making it very interesting to observe.
If you provide it with fine substrate, clean water, enough space, and a varied diet, it can be a long-term attractive inhabitant of the aquarium.
Insulated aquarium in the basement. Due to the fact that the basement is not heated at all and temperatures in winter are around 3 degrees, I have completely insulated it. The shelf is insulated on all sides with 3cm polystyrene, including the doors. Surrounding the aquarium is an additional 5cm of polystyrene. Each aquarium has a heater. Inside, there is an LED light + an air compressor - the heat generated from the LED light and the compressor is utilized inside and heats the space around the aquariums.
You are invited to the traditional autumn congress of the Czech Cichlid Club, which will take place on Saturday, November 14, 2026, in Žďár nad Sázavou at Hotel Hajčman. Look forward to lectures by Mgr. Zuzana Musilová, Ph.D. and Ing. Miloš Kroupa. The members' assembly, a rich raffle, and an evening social event full of inspiring discussions. Don't miss this opportunity to meet enthusiasts and gain new knowledge!
You are invited to the traditional autumn 29th Congress of the Czech Cichlid Club held in Žďár nad Sázavou at Hotel Hajčman.
Program of the day:
10:00 - 12:00 members' assembly of the Czech Cichlid Club - for members of ČCK, non-members, and those interested in membership
13:30 - approx. 16:30 lecture afternoon:
Mgr. Zuzana Musilová Ph.D. – Cichlids in the crater lakes of Cameroon
Ing. Miloš Kroupa – Diseases of aquarium fish with an emphasis on cichlids
&
juicy RAFFLE for those who endure… and for those who endure even longer, a social buffet and party
Everyone is warmly invited. Raffle and pleasant atmosphere guaranteed.
VENUE OF THE CONGRESS:
Hotel Hajčman, Strojírenská 372/11, 591 01 Žďár nad Sázavou
https://maps.app.goo.gl/umippwXKAgkZMkEF7
