When a crayfish is not walking, how does water move over the gills?

This is one of the reasons crayfish almost exclusively live in (fast) flowing water: when the water moves they don't have to. Since their gills are located on their "legs", they just have to wave them around a lot to breathe, which they indeed do.

The following paragraph is about insects, but it should be similar for arthropods.
"However, water surrounding the insect can become oxygen-depleted if there is no water movement, so many aquatic insects in still water actively direct a flow of water over their bodies. They flap alot."

https://en.wikipedia.org/wiki/Crayfish
https://en.wikipedia.org/wiki/Decapod_anatomy
https://en.wikipedia.org/wiki/Gill#Invertebrate_gills