Tank size - let's start with the size of the tank. Of course, it depends on the size of the fish and their quantity. Most of the common species available in Polish waters reach truly impressive body sizes - from several dozen centimeters to even a few meters. As a result of conscious breeding - in order not to tire our wards - I suggest to keep only specimens in an artificial environment, which maximum size is up to 30 cm. In that case our aquarium should have a minimum size of 250 liters. Of course, larger is better - maintaining biological balance is easier in a larger tank, so the fish are healthy and satisfied.
No heater - cold water aquariums don't need a heater - the average temperature in such a tank should range from 15-20°C (if we are not going to imitate individual seasons or hibernate fish). In Polish conditions, it is enough to place the tank in an unheated space, not exposed to drafts or sudden temperature changes. The situation gets a bit more complicated in summer, when the air temperature often jumps above 30°C. The solution could be to use professional aquarium coolers purchased at a pet store (unfortunately, they are not cheap devices) or to make your own heat exchanger using a car radiator, a travel fridge or other equipment (there is no lack of advice on the Internet). Another solution, a bit troublesome, is to regularly change part of the water for cooler water, every 2-3 days, depending on the needs.
The problem of "sweating" panes - With a large difference in temperature between the environment and our tank (its water temperature), a natural process of condensation of water vapor will occur on its walls from the outside. This problem can be dealt with by adequate ventilation of the room/aquarium or by mechanical, systematic removal of condensed vapor (which does not solve the problem).
Water aeration - this is a very important factor in cold water aquariums and is absolutely required - especially if we decide to keep fish from flowing waters. The water must be heavily aerated at all times. To this end, you can use an appropriate number of plants (we obtain oxygen in the photosynthesis process - the method works only when the lighting is on), water filters, various types of air pumps in combination with appropriate aeration stones or strips - in addition to causing water surface ripple, which facilitates the permeation of oxygen. You can also use a rain shower.
Water circulation - when we want to replicate a flowing cold water reservoir (e.g. river, stream, etc.), creating an appropriate water circulation - its current, is an important issue. Water currents are crucial in stimulating fish's lateral line, which reacts to the smallest changes in water pressure (water movement). As a result, frequent changes in water circulation, by creating appropriate currents, swirls, acceleration of the current or its slowing down, seem to be crucial, especially when in the future such fish are intended to be released into the wild.
Lighting - in the case of a cold water aquarium, sunlight - natural lighting, is often used. However, creating optimal conditions with its use is not easy at all. Sun rays can contribute to violent daily differences in water temperature between day and night, especially when the tank is exposed to direct contact with the sun. Such fluctuations are not good for fish, which can start getting sick in a short time. A more convenient and safer way seems to be the use of not too intense artificial lighting, which we will be able to regulate according to individual needs (depending on whether we have plants or not).
Algae - algae develop in every water tank. Their overgrowth is caused by a disruption to the biological balance in the aquarium. If such a problem occurs, the cause of their formation should be removed first and foremost. The solution to removing excess algae may be tedious mechanical cleaning of walls (using a scraper) or stocking with species of herbivorous fish, especially those that consume algae.
Fauna and flora - The primary question every future owner of a coldwater aquarium asks is - where to get fish and plants? In pet stores, we won't find native species (they aren't popular enough), and catching from natural habitats may not be legal. It turns out that in Poland at least 14 small species of freshwater fish are under strict protection. On top of that, there are regulations concerning catching in particular reservoirs, season, breeding times, etc. The safest way to obtain material in this case seems to be from private/breeding tanks, which are drained at certain times (usually in the fall). However, obtaining fauna and flora or decorative elements in this way carries a certain risk of bringing in various unwanted, pathogenic parasites, viruses, and bacteria. Therefore, the material should always be quarantined or disinfected. Another important issue is properly matching our charges. We pay attention here to the size of the fish, their temperament (for example, we do not mix predators with potential victims), their requirements in terms of their numbers (school fish, living alone), etc.
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