bam Demanding level

Shallow grassy area at Mariager, Mariager Fjord

Sponsored by

Denmark, Nordjylland, Mariagerfjord kommune

For this aquarium I have realized an old dream. I have made several biotope setups, all of them from exotic places far away. Non of them from places I have actually been able to research myself.

For this one I have created a habitat that is located very close to my home. I have been able to collect both fish, plants and stones from the original habitat. This has been a very interesting proces for me.

I have chosen to recreate this area since it is located by two different species. The original habitat is mainly inhabited by juvenile Gasterosteus aculeatus, and that is why I have only juvenile specimens.

Submitted by
David Nørholm
Approved by
Gireg Allain & Aleksey Malyshev
56.6548271, 9.9866066
Geographical region
Northern Europe
Drainage Basin
Mariager Fjord
River catchment
Mariager Fjord at Mariager
Water body type
Water body name
Mariager Fjord
Water body part
Water body course
Lower course
Water body: tributary of
Tributary name
Mariager Fjord

Videos above and below water

Water Chemistry

Water information

Water type
Brackish water
Water color
Clear water
Water transparency
Concentration of sediments
Water temperature
27 °C
Water flow/curent

Chemical parameters

8.0 mg/l
7.6 mg/l
Dissolved Oxygen

Aquarium information

Aquarium description

Set-up date
May, 2022
Aquarium decoration

All stones are locally collected in the Mariager Fjord, at the very spot, where I also was catching the fish. A few of the stones still have that layer of algae that dominates most of the stones in the fjord. The plants are also locally collected, just next to the stones.

The substrate I had to recreate by using two different types of sand. I was not able to use the substrate from the fjord, since this was simply too dirty and dusty. I have only used small amounts of silt, also collected locally.

Furthermore I have used a couple of old shells, which is also represented in the habitat.
I have not used any driftwood, since this is only rarely seen in the fjord. It is often carried on to the coast by waves.

Aquarium equipment

Filtration: black Hamburger Matten Filter, powered by a Pico 1150 powerhead. The light is dimmed LED, to recreate the rather doll light conditions in the fjord.

Fish care

This setup is relatively complicated because of the conditions in the fjord. Mariager Fjord has a lower average temperature than the typical aquarium. Danish inlets only rarely reach temperatures higher than 25°C. However, the shallow water areas are an exception, since these can reach a few degrees higher during the summertime. Nevertheless the setup requires very frequent water changes with cold water to keep the temperature in balance – especially with summer temperatures close to 30°C (even indoor).

Furthermore the setup is complicated even more by the fact that Mariager Fjord is brackish. Therefor the water, that comes with the frequent water changes, needs to have the right amount of salt to stabilize the salinity of the aquarium.


  • Gasterosteus aculeatus (Gasterosteidae)
  • Pomatoschistus microps (Gobiidae)
Plant care

The plants of the tank is Ruppia maritima and they do not need any special treatment. Ruppia maritima is a very strong plant, which all of the few plants thriving in Mariager Fjord also needs to be, since it is a quite hostile environment for any plant.


  • Ruppia maritima (Ruppiaceae)
Water care

20% of the water in the aquarium is changed every two days. Furthermore I am using a refractometer to help dose the salt correctly.


80 cm
35 cm
35 cm
98 L

Substrate in aquarium

Stone form
Submerged terrestrial vegetation

Comment by the expert

Gireg Allain: Unfortunately, one important factor is missing in order to reconstitute this biotope in an aquarium: the concentration of salt used. It is this parameter that makes this environment a unique biotope. The aquarium is very well made, the mix of plants and rocks is well made. But the decor is a bit unbalanced. It must be said that to recreate in an aesthetic way this type of flat and desert biotope is extremely complicated, it was a very nice challenge! Perhaps it would have been more spectacular to do it in a square, shallower aquarium.

Aleksey Malyshev: The aquarium is decorated in accordance with the description of the biotope. But why is the sand in the aquarium white? Or is it such a video (a trick of the light?)