as i was reading this thread i saw several posts regarding the need to change the water every few days to keep the water clear as well as a statement about flowers lasting longer in styro. Makes me cringe when i read it.
If you use a good preservative with a water clarifier, such as aquaplus... The difference is clear, you will not need to change the water every few days. Provide your customers with the little 5 or 10 gram packets and just have them top off the water. Not only will your water stay clear, but because you are also feeding the flowers, they will last longer too.
A longer lasting arrangement makes a more satisfied customer.
Here is a link to a quick guide to care and handling.
http://www.syndicatesales.com/aquaplus.asp