If you are sure your filter is working then perhaps it is the "muck" that is in the water. I live between Ampolla and El Perelló where the tap water is undrinkable and is often brown. Where do you live and is your water like mine? Does your kettle "fur up"? If it does then the filter is not working. Does your "gardening" water go through that filter?

Re "right plants". The list is endless. Well nearly.
As Nigel said a year ago forget virtually anything from GB and places like Bauhaus and plenty of garden centres which I cannot name.
Places that sell "Mediterranean" plants include:
Culti-Delta (Amposta) They will send you a list of their plants if you ask for it.
www.salagraupera.com/ near Mataró. Has a good web page in English too.
www.carex.cat in Banyoles is fine also but their catalogue is only in Catalan, I think.
It is no use asking for plants with English names like "daisy" or "buttercup" you have to have the Latin name and it is always at least two words. eg Leucanthemum vulgare (daisy) or Ranunculus acris (Buttercup). Out of interest all buttercups are not yellow. I have taken photographs of red and white ones and have photographs of pink, orange and varied coloured ones.
The plants that these people, and there are others, sell should be planted in Autumn (not Spring) and will need watering for the first year and maybe two (depending on the weather) and not every week either, but should be OK after that.
None of the sellers will guarantee success but their material has a far greater possibility than most plants that grow in GB. The reverse applies in that, what grows well here will not normally do too well in GB.
The plants that I have in my garden (Nigel will tell you the same about his (or his wife's) garden are not watered until things become desperate and are going to die unless given a drink. Desperate means at least one month in the middle of summer with no rain as well as showing signs of distress.
Nobody waters the plants that are on the hillsides behind you. And nobody gives them fertiliser or compost either. They don't need it. And they don't like it.
Remember that lots of mediterranean plants "hibernate" in summer. They are smart. If you look around you will notice that there are not a lot of plants in flower in the middle of summer. Most local plants are Spring or Autumn flowering.
Hope this helps
Bricon