I used to run E&F and never had any problems with salt build up, but I did run out of room for my roots. You should change your res once a week or possibly more, since it is only 6 gallons which is not a lot of volume to work with. The plants use different nutes at different rates so you need to keep it fresh for optimum growth. I found I could let my veg plants go without a res change for about 3 weeks with little to no negative effects, but in flower the effects became noticable on day 10 of the same 12 gallon res, and nutrients started getting locked out.

As for when to use your flower nutes, I used to mix half grow and half flower nutes for the first two weeks and then only flower nutes from week three to 7, then flush for 5-7 days. Good luck! :thumbsup: