Water bill question

Tell us,
The consumption (should be in the bill) ,the period , how many persons are included in the bill.

You may have to give a composition de menage to the supplier, if the bill didn't include all three of your family- This makes a difference.

I have a water bill to pay for the quarter to the end of July. It is for €30.00.
I live alone outside Brussels.
This is about standard for me for the last 5 years though, as a pensioner, I am very careful with my consumption.
How did your supplier arrive at your bill? Is it by their rep reading your meter? Is it by you sending them a meter reading? Is it based on the consumption used by a previous occupant of the place?
You should definitely confirm that the reading on which the bill is based and the current meter reading coincide.

Our last annual bill is for 450 EUR. We are a family of 4 in Brussels. We also have a slightly leaking toilet, but to repair it will cost a lot as we have to remove tiles from the wall. It's a rented appartment.

€600 does seem excessive.
It sounds like this may be your first bill?
Ask the water company for a complete breakdown and maybe also to check the apartment. It wouldn't be the first time that I have heard of someone paying for someone else's water.
There could also be a leak somewhere that you are unaware of, again this needs checking.
We are a family of 4 in Ixelles, bill is less than €300

Our water bill is around €200 a year - 4 people (2 of which teenagers) 3 toilets - 2 washing machines. (We do have a rainwater cistern that covers some of the toilets and washing machine useage)

First, as others mentioned, check that it's not a mistake. Take the reading of your water meter and compare to what the water company says you consumed. They got my numbers wrong on more than one occasion.

Second, I'd check for leaks. Even a slightly leaking toilet can quickly get very expensive. I learned that the hard way: €800 for 6 months of normal water consumption.