why change the water dwc?

as above im just setup a dwc with 1 week old seedlings i will be using canna aqua why do you need to change the resivoiur every 2 weeks cant i keep just add nutes and/or water when needed and not change every 2 weeks

Cuz ur plants only use some of the avalble nutes. Jus because its avalible doesn't mean they need it. So if ur nutes has 10 dif sources of nitrogen but only uses 2 of them, then how u gonna add jus the 2 it needs with out the other 8 becoming too much and toxic

Pour the old res water on house plants or in your garden. But def change it.

I ignored changing my res's the first year and all my harvests were kinda small and my buds were not very big. After that, I changed the resevoirs every 2 weeks, except at the end of weeks 6, 7, and 8. I changed these intervals after 1 week as the Fox Farm feeding formula calls for nute changes at the end of each of those weeks. Also, week 6 is not only a resevoir change, but also a flush with a clearing solution in the water, so you literally change the water TWICE that day. I add GH celaring solution to my water and let the system run for 2 hrs then drain again and refill with the Week 7 feeding levels.
The flush clears excess salts and nutrient build up from your roots.....making for vigorous feeding in the flowering stage also.

To clarify, by weeks 6, 7, & 8, I am not referring to flowering weeks. I am referring to overall plant cycle, making these change times actually the 2nd, 3rd, & 4th weeks of flowering.

And remember, pH test your water daily. If your pH gets horribly out of control, drain your resevoir and and start with the correct pH.