Will fertilizer mixture deteriorate over time if mixed in advance?

Hi mate, it returns closer to a PH of 7 over time. You're fine to adjust PH with nutrients in. I do it all the time and it has to be done in dwc. If you're going to be leaving it a week I'd reccomend getting an airstone and keeping in the dark if you can. The airstone keeps it fresh and keeps oxygen in the water. Making it difficult for harmful bacteria to form. Keeping it in the dark also makes it harder for the bacteria to form. If you do all of the above then you can leave it a longer than a week. People run the same res in a dwc for multiple weeks. Hope this helps and good luck with the rest of your grow 🙂