I use "water Irises" of different species. I like the purple and blue flowered ones. I would give you seeds or starts, but mostly that is not possible trans nationally. I also use Juncus species. There are native Asters in this area that seem to grow in moist roadside ditches. These all have seeds this time of year where I live in Washington State USA. I guess what you would need are Facultative Wetland species.
I have noticed that with such areas of my own I need to add more small stones or gravel to low spots to prevent water puddles which would create mosquito breeding spots.
One might be able to collect seeds or notice a ditch clearing road project where "weeds" are being dug oiut and thown away so take home the "weeds" and plant them.
I did a Google search like this:
"small wetlands of Castellon, Spain"
that had many interesting results for me, including some constructed wetlands for various purposes.. YRMV
"constructed wetlands Castellon, Spain"
many good results possibly close to you and you might quietly ask for seeds or starts at one of these projects.
and another google search:
"small wetland plants of Castellon,Spain"
and many useful results in addition to this turned up..."Ethnobotanical review of wild edible plants in Spain" https://www.researchgate.net/publication/227510655_Ethnobotanical_review_of_wild_edible_plants_in_Spain
The paper here: https://www.researchgate.net/publication/40424182_Soil_Requirements_of_Three_Salt_Tolerant_Endemic_Species_from_South-East_Spain
has references at the end of the paper that could be interesting for you also. Soap by definition is a salt, although the dish soap might be a detergent.
Finally. While the writing is somewhat pie in the sky for my taste, Anna Eddy
has produced some of the most interesting publicly accessible work on water filtration as she documents in her book "Solviva" which I have read. I borrowed it from our local library. If you had money you could purchase a copy. I have not read her newer book “Green Light at the End of the Tunnel”, but I would like to. The main reasons I point to her work is that it is written clearly, approaches problems with down to earth common sense, and is not afraid to detail failings of her own and of "the system" we are all so familiar with while at the same time she perseveres and finds compelling solutions. Like growing onions in urine water!!! Other practitioners of waste water management seem reluctant to divulge their methods, the species they utilize, or even patent their "system".