Ok, here goes:
First of all it is important to know wether you are an owner or a resident in the building. ( Makes a difference in the approach of appeal)
At the moment I'll assume your are an owner. Were you present at the meeting where the decision to install a water softening system was made or did you just get the conclusions of the meeting. If you were present at the meeting did you voice your concerns and asked to be noted as opposed to the proposition in the conclusions of the meeting?
As an owner you have the right to appeal every decision taken by the yearly convention of owners through a request to the local judge of the peace.
This right is guaranteed by article 577 9 §2 of the civil code.
Art. 577-9. <Ingevoegd> § 1. De vereniging van medeëigenaars is bevoegd om in rechte op te treden, als eiser en als verweerder.
Iedere medeëigenaar kan echter alle rechtsvorderingen alleen instellen betreffende zijn kavel, na de syndicus daarover te hebben ingelicht die op zijn beurt de andere medeëigenaars inlicht.
§ 2. Iedere medeëigenaar kan aan de rechter vragen een onregelmatige, bedrieglijke of onrechtmatige beslissing van de algemene vergadering te vernietigen of te wijzigen.
Deze vordering moet worden ingesteld binnen drie maanden te rekenen van het tijdstip waarop de belanghebbende kennis van de beslissing heeft genomen. De medeëigenaar die op regelmatige wijze is opgeroepen, wordt geacht kennis van de beslissing te hebben genomen op het tijdstip waarop ze door de algemene vergadering is goedgekeurd.
Honesty obliges me though to inform you that your chances to win such an appeal, if the vote was held correctly, are marginal at best. Softening of water is in most communities in Belgium a real necessity and more and more often happens at a regional level. As far as is known after decades of use no health risks have been detected on short or long term. The only possible problem with a water softening installation is if the system is badly managed or not regularly serviced but that is the responsibility of the blockkeeper. If such problems should arise ( high sodium levels for blood pressure patients) health services should be alerted inmediatly and the system recalibrated.
As such you would have no case to appeal an infringement of your basic human rights.
Living in an appartmentbuilding automatically means you live in a community and regularly in a community the demands of the many outweighs the demands of the few.