I am not a regulations guru. I also don't know whether Anker Solix implicitly forbids this. My thoughts on this: A more or less "fixed installation" via a cable reel is never good, because of the installation regulations and because a current-carrying cable generates heat and a cable in coiled form cannot dissipate the heat well. The cable then heats up and the cable reel can overheat. If the cable still has damage to the insulation, this can lead to undefined short-circuit currents, which further favour overheating.

An alternative idea would be to buy a good, clean extension cable for outdoor applications (e.g. https://www.digitec.ch/de/s1/product/brennenstuhl-verlaengerungsleitung-ch-230v10a-10m-h07bq-f-3g15-orange-kupplung-t1310a-stecker-t1210a-18760154) and then connect a multiple socket https://www.digitec.ch/de... , then the "installation" is much better / safer in terms of heating.

I'm not an electrician either, but this is how I interpret it. If the Anker Solex is outdoors, it is mandatory to fit an adapter that does not comply with IP55. A Schuko plug with IP55 or higher, on the other hand, is permitted; fit a Swiss plug at the other end (bearing adapter).
If the Anker Solix is installed indoors, I see no reason to use the adapter.
I assume that your aquarium is indoors. The shortest possible 1.5mm2 cable with direct sockets should not be a technical problem.
A 1.5mm2 cable can be loaded with almost 3000W. The Anker Solix only delivers a maximum of 1000 watts.
The cable reel must always be unwound and laid out under heavy loads, but this is not a problem in this case
If you are not allowed to connect multiple sockets, you should no longer connect a cable drum to a mobile power generator.
Do not plug in and operate an emergency power generator on a caravan or motorhome.

No, a multi-socket is not allowed and I find that absurd.
Otherwise I'm very happy with it