is a seven stage ro/di overkill?

howdy folks!



so, as I'm getting my tank up and running (I added my. Salt and waiting for that to mix, then my buddy and I are gonna start planning for the base rock placement so we can get a cool star wars theme going!). And I have been reading up on ro/di units. I am seeing one on eBay that is a seven stage process for $120. However, I see a 4 stage process for $80, effectively saving me about $40 bucks. My question is, is can I get away with 4 stages? Are seven stages overkill?



please provide me with some insight!



thanks dudes!'