Please try my idea for improving rooting in water. This idea works with other plants so it may work well with dragon fruit plants. Add an airstone into the water and use an aquarium air pump to add more oxygen into the water. In this bacterial growth will be minimal and there is no need to change water at all. Roots should grow a lot quicker this way. This idea of using airstone is behind simple hydroponic gardening setup at home. I tried this with other plants and it worked really well when I did root lots of stems together in one go. I am doing an experiment right now about rooting dragon fruit plant and grafting at the same time. Normally, people would root them first and wait until they start growing before grafting. This whole process could take 3-5 months! I think it is possible to cut this down to 6 weeks by grafting and rooting at the same time. I am doing this experiment right now but hopefully others try it too so we can compare. The climate in my part of the world rigth now is autumn and it is getting cold so the chance of success might be lower. It would work better in spring and early summer. In my current experiement, I graft a scion on an unrooted host stem. Then I put it into a pot for rooting. I also put a clear pot up-side-down on top of the rooting pot to create a mini greenhouse for the cutting. I also add a piece of wet paper towel in to add extra humidity into the setup.
