I use a portable RO/DI unit/ I have great well water that I need to lower the pH and TDS of for fish that need this. I also batch water for use in humidifiers used by my brother. As important is the fact that I can take the nit with to fish events where I am selling. I have all the adapters available and lots of hose as well.
I do not know if the company from whom I have bought two 75 gpd units sells outside of the states. They make all sorts of units from small ones like I use to much bigger. This is the model we have. It has a carbon, an RO and a DI module. If I had it to do all over I might have a sediment module rather than the carbon, However, when new my system puts out 0 TDS water.
I cannot have the system permanently installed. But I have a fish space I carved out of a storage space and I had a utility sink installed. I use the garden hose hookup and the sink faucet actually has a splitter to create two independently controlled outputs. One has a hose permanently hooked up and the iother feeds into the sink. I hook into that second output when I need to make pure water.
One important consideration in this sort of system is that, once put into use, the RO membrane must not be allowed to dry out or it is done. With my model I can buy replacement cartridges as needed.
Finally, to monitor what I am doing, both for the tank water and the pre-batched water I use for water changes (I mix RO/DI and my tap in the ratio of 11/9), using this piece of equipment which is actually about double the price of the RO/DI unit.
https://www.amazon.com/Temperature-...urements-Calibration-Hydroponic/dp/B0081K32B0
I also have a hand held TDS meter
https://www.amazon.com/HM-Digital-TDS-3-Handheld-Carrying/dp/B000VTQM70
I cannot say if the above two items are available in Greece, and if they are, at what price.
I run the Guardian monitor on the tank itself and I batch my changing water by the tank. So I am able to move the two probes from the monitor into the can in which I prepare the changing water. Between water changes the parameters tend to drift up a bit and I bactch the changing water to parameters designed to get the tank back to "normal," i.e. the targetted parameters.