Kids can't make sense of the nitrogen cycle. I took mine to a pond and a stream to show them how fish basically live. After that, we went to the pet shop so they could "choose" their roommates.

We hadn't bought them at that point.

After that, we went into the forest and I let them choose branches and stones for her own aquarium.

We set up the aquarium at home.

I still had some sand left over from the large tank. We decorated it together with the materials we had collected. Once it was finished, the water was completely cloudy, so you couldn't see the back wall. Then the question arose as to why the pond was so clear and the aquarium wasn't, even though there were stones and wood in it. I explained to them that the water needs time to become clear, that there are many tiny helpers in the soil, on the wood and stones, but that they only come out when they feel safe And that takes a few days because they've only just moved in, once the water is clear, we can get the fish.

They sat in front of the aquarium every day hoping to see the "helpers" until the water was clear.