Outrageous water bill

You could try discussing it with the landlord but I doubt it will change anything. In rented apartments it is the tenants' responsibility to keep an eye on the facilities you use daily (toilet, boiler, etc) and ensure they are maintained. In fact many landlords in Belgium would have turned round and told you to fix the toilet yourself. It's kind of hard to believe you could not have spotted the leaky toilet because either there would have been water all over the floor, or a lot of dripping with water going through the cistern all the time.