If you have not already done this, try removing ALL the green leafy stuff that would be below the water line. This decays faster. You only have to add just a touch of bleach to the water to change the PH. You don't have to use enough where it would cause damage if spilled. Some say that putting a penny in the water will keep water fresh. I have not tried this. (Make it a habit never to pour old flower water down the garbage disposal opening. You could accidentally pour the penny down there and not notice and the next time you turn the disposal on you wreck the thing.)

It may just be the smell from the water that you are smelling and not the decaying flowers. Does your tap water have a slight odor to it after it sits in a glass? Try using only rain water for cut flowers. I do this.

If you are using flowers from you garden keep in mind that some flowers are practically meant to be used for cuttings and some are just not suited. Some do deteriorate faster than others.

There is a whole forum here on growing flowers specifically for cuttings. They might have better tips than I do.

You only need 1/4 tsp. bleach per quart of water. A tablespoon of white vinegar (along with the bleach) and a tablespoon of sugar per quart of warm water helps the flowers last longer. Most commercial cut-flower preservatives like Floral Life that you get with bouquets from the florist or supermarket contain an antibacterial agent, an acidifier and some kind of sugar. Dissolving the preservative, whether homemade or commercial, in hot water and allowing it to cool to lukewarm works best.

The penny trick does nothing unless you have an old solid-copper penny (pre-1980). Pennies now are almost all zinc with a very thin copper coating. All the copper does, though, is acidify the water, which can be accomplished just as well with a bit of vinegar.

Aspirin doesn't do anything either.

NEVER use softened water for cut flowers. It contains sodium which makes the flowers wilt and rot.

(My mum was a florist for umpty-ump years.)

"The penny trick does nothing unless you have an old solid-copper penny (pre-1980). Pennies now are almost all zinc with a very thin copper coating."

Good point. They don't make 'em like they used to.

Cut your flowers in early morning before the sun is high. Remove all the foliage save the two top leaf sets. Cut the stems on a 45-degree angle under water. Cool off with icy water. Cut the stems again, about 1/8 inch and run hot water over the cut area only. Place in a refrigerator immersed in cold water only if you have a refrigerator that you use for nothing but ice, sealed condiments and soft drinks, etc. If there is food, it can contaminate your flowers and cause them to ripen too soon. After refrigerating for a few hours, place in a vase of cold water to which you have added 1/4 teaspoon bleach to a quart of water. Change your water daily. Remove flowers when they begin to get a putrid smell or the water starts becoming cloudy. You can add aspirin if you want the flowers to last to a process of rooting. Willow water, rooting flowers. I add aspirin to zinnia and marigold water to root them and put them back in the garden in mid summer.