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init.lua

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+--TODO: other physics-speed-manipulator(e.g. crystallshoes) manipulate but it not work because it is all the time at 0.4 or 1. But you should walk only 40% of normalvelocity.
+minetest.register_globalstep(function(dtime)
+for _,player in ipairs(minetest.get_connected_players()) do
+
+  local name = player:get_player_name()
+  if minetest.get_node(player:get_pos()).name == "default:water_source" then
+    player:set_physics_override({
+            speed = 0.4
+        })
+  else
+    player:set_physics_override({
+            speed = 1
+        })
+  end
+end
+end)

mod.conf

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+name = slower_in_liquid
+depends = default