It's "acclimate" not accumulate

. If the water properties of the 14G and 17G are exactly the same then no.. you do not have to acclimate them. However.. if the readings are off in the 17G even by one or two points.. acclimating them would be ideal for the overall health of your livestock.
Also, the brown stuff you see on the sand is diatoms. Their appearance is part of the whole "new tank" thing. They feed off of the silicates in your tank. So if you use RO water, they will eventually burn themselves out and will disappear. Some times it takes days, other times it takes weeks. For me it took a week total for them to be completely eradicated. However, if you are using regular tap water in your tank, and the tap water has silicates present.. then you won't ever get rid of them. They spread rapidly even off of minuscule amounts of silicates. And if you are using tap water.. diatoms are going to be the least of your problems down the road.. honestly.