I didn't wash mine and it worked fine. Most people think of washing as they have done with gravel and that is a big NO. Digging your hand into it and mixing it up will break down the little pellets of compressed dirt and turn it to mud. I went straight from bag to tank. Good Luck.
I dump it in and put fine sand on top then planting ,never wash it and it is fine.You might need to wipe off the dust bubble around tank after fill water up.The reason I add a lay of sand on top of it just because this kind of substrate is a bit lighter ,wont hold plant down good enough.If you plan to use this substrate for shrimps.The pellets will be not good enough for baby shrimps to hide after while grown .I use the bushes kind of plant for them to hide instead.Any way it will be come mud after 6-8 months or longer ,depend on how many plants in tank use its fert.