Winter: what must be brought in at all costs?

The low temperatures are coming, after a rather mild autumn.

Of course, the tropical fish that had been taken outside to the garden for the summer have been brought back inside since October, depending on the region.

 

 
You found them more colorful, more beautiful, livelier, and even wild again! Sometimes, you also found them more numerous.

Some species that are not very sensitive to cold will spend the winter outside, like goldfish or the famous Tanichthys for example, or others in favorable regions.

 

But the cold is coming, and the question arises for plants and invertebrates.

 

 
I personally let Neocaridina davidi shrimp spend the winter outside in the Paris region, and they came out looking superb. A buried tank keeps the water at the bottom at a milder temperature, allowing many species not to suffer too much from the cold.

 

Water lice love the cold and can stay outside all winter, even running under the ice. They will be even more prolific in spring, as they prefer frost to heatwaves.

Ostracods and Blackworm will enter a kind of lethargy, waiting in the mud at the bottom for the return of spring.

 

 
Likewise, daphnia will become less numerous, with a few rare individuals surviving under the ice. In spring, the population will restart from these survivors, but also from floating eggs laid in autumn, capable of surviving anything for a very long time!

It is therefore wise to bring in some daphnia at 18 or 20°C if you want to continue breeding them for your fish during the winter months.

 

Plants, on the other hand, are quite surprising. Some pretend to die: duckweed and large duckweed, azolla and a few others disappear completely from the surface, surviving at the bottom of the water in a dormant form. This is how you will see them reappear in spring, as if by magic. Azolla does better: it sends dormant forms to the bottom while often maintaining itself on the surface as well.

The three-lobed duckweed, meanwhile, takes advantage of the disappearance of its competitors to form beautiful green cushions on the surface, insensitive to frost.

 

 
Others, like Salvinias or frogbit, really cannot stand winter and die permanently after severe frosts. If you do not bring a few specimens inside to warmth, you will lose them!

 

Irises, water mint, or Purple loosestrife do not fear winter. They will regrow from their roots as soon as the fine days come, each year a little stronger. Water mint even takes advantage of winter to spread, its creeping stems resisting everything.

 

More surprisingly: Sagittaria subulata and Vallisneria, reputed not to withstand cold, spend every winter at the bottom of the water under the ice layer in my care. Their leaves are shorter to avoid the surface ice, but they do not seem to suffer from the cold at the bottom and multiply over the years and seasons. This behavior, totally contrary to what aquarium manuals say, is nevertheless consistent according to my observations.

Finally, Ceratophyllum and Water milfoil, even Najas (which is more surprising), spend the winter in a trash pond or basin without flinching. It is only just that Cerato changes appearance (shorter and denser strands) and that Najas becomes a bit more brown-red.

Climate change, which makes mild winters more frequent, will probably only increase these resistance phenomena of species once thought too sensitive to cold to spend the winter outside.

 

But bringing them inside to an aquarium in winter, if you prefer this option, allows them to continue growing and greening all winter, waiting for the return of fine days.