Sorry I forgot to say yes you can flush with plain water, that's fine. Like I said only for salt build up, which should never be allowed to happen anyway. But immediately feed straight after with a nute solution so you don't upset the cation exchange.
I've mentioned this before so in the spirit of actually explaining it a little, Cation Exchange Capacity is in simple terms how much your organic matter (coco) can hold onto elements/cations (nutes) before releasing them to the plant, or rather over flowing so to speak and becoming available.
The surface of your coco contains negatively charged surfaces which attract positively charged cations. Your coco needs 'charging' with the desired cations I.e Calcium, Magnesium etc but not all coco comes pre buffed. That's why you pre buffer your fresh coco with a nute solution somewhere low in the PPM's because the coco needs time to 'attract' the cations (nutes) and build them up ready for your plant to TAKE them when it needs them.
If you water plain and don't replace the cations immediately then by the time you've figured out what's happened, you feed, then the coco might need 24 hours or something to 'recharge' because as you feed fresh coco more and more the exchange rate slows until it reaches an equilibrium. You can get fully buffed coco but it means you don't go through the process of the nutes releasing really quickly and slowing down while it buffers.
So anyway it might mean your plants can't access the nutes in the coco even though they're 'there', cos you've let the plain water leach out the nutes and now the negative surfaces need to attract positive cations all over again and build them up cos coco holds onto nutes really well.
Edit: @EventHorizan @Wisher619 please read this post of mine and tell me if this is a good articulation of CEC, because...y'know...remember that time? :D