For Wishiwashi's "Scatter" Ability, when does this check for damage counters happen, before "in between turns" when Poison counters are placed or afterward?
After your opponent concludes their end of turn actions including all effects of attack and cleaning up any KO, then before in between turns you check for any damage counters on Wishiwashi that would trigger Scatter to shuffle Wishiwashi and all cards attached into your deck.
Scatter