Tap water in most countries is allowed to have some nitrates and the maximum allowable level is generally around 40ppm. However, nitrates are bad for anything that ingests them (except plants) and you want the level as low as possible for yourself, your pets and your fish.
Some bottled water is fine, some isn't. You need to check the ingredients and avoid water that contains sodium chloride (salt). They usually have sodium and chlorides listed individually rather than sodium chloride on the same line. You can check the pH, GH, KH, ammonia, nitrite and nitrate of bottled water and if it is suitable for the fish being kept, then you can use it.
The other options include a reverse osmosis unit that will remove most nutrients and minerals from the water. There are filter additives that remove nitrates from water. Growing live floating plants in the aquarium ca help to remove nitrates. Some people grow terrestrial plants above the tank and let their roots grow into the water. the terrestrial plants can use a lot of nitrates. Distilling water is another option, which can work well in hot climates.
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SOLAR STILL
You can make a solar still, which would probably work well in Greece. It would give you pure water, no waste water and be free to make pure water, it just requires a bit of sunlight.
Use food grade/ food safe containers for this.
Get a large plastic storage container and put it outside in the sun.
Pour a bucket of water into the storage container.
Put a clean bucket in the middle of the storage container. Have a rock in the bucket to stop it floating around.
Put the lid on the storage container.
Put a rock or small weight on the lid in the middle, so the lid sags a little bit above the bucket.
As the sun heats up the container, water will evaporate and condense on the underside of the lid. The water will run towards the centre and drip into the bucket. When the bucket is full of water, you put it into a holding container and put the bucket back in the storage container with another bucket of tap water.
You get pure water with a pH of 7.0, 0 GH, 0KH and no wasted water, no power used and it's cheap to set up.