How can I lesson the plant water from becoming rancid?

Question

These are plants I have only in water. The water starts to smell bad. Is there anyway to stop that?

Answer 1

The water stagnates and because even though you have live roots in the water (I am presuming) or even the whole plant there is always the process of decay. Therefore it is important to change the water frequently.
So there is no way to stop the process of decay but if you change the water frequently you won't get the "smellies".
How large a reservoir are you growing these plants in?

Answer 2

Yes; carefully changing the water once a week or so is the answer.