if i leave my water change water standing for a week

will this make any difference if i just add the stress coat before i add the water when i make water changes, i siphon out ten gals for my freshwater 55 gal african cichlid, and i siphon out 8 gal for my 30 gal brackish green spoted loner puffer, will it be better if i leave out an 18 gal condatainer running with an air pump every week before and after water changes, so is this in any way equal to ro di osmisis water, i still use the stress coat, and always mix any salt before actually adding it and let it stand for 30 mins, is there any way of never having to perform water changss???

 

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