Uppercases slot defines.

code/game/objects/items/tanks/tank_types.dm

@@ -137,7 +137,7 @@
 /obj/item/tank/internals/plasmaman/belt
 	icon_state = "plasmaman_tank_belt"
 	item_state = "plasmaman_tank_belt"
-	slot_flags = SLOT_BELT
+	slot_flags = ITEM_SLOT_BELT
 	force = 5
 	volume = 6
 	w_class = WEIGHT_CLASS_SMALL //thanks i forgot this
@@ -157,7 +157,7 @@
 	desc = "Used for emergencies. Contains very little oxygen, so try to conserve it until you actually need it."
 	icon_state = "emergency"
 	flags_1 = CONDUCT_1
-	slot_flags = SLOT_BELT
+	slot_flags = ITEM_SLOT_BELT
 	w_class = WEIGHT_CLASS_SMALL
 	force = 4
 	distribute_pressure = TANK_DEFAULT_RELEASE_PRESSURE

code/game/objects/items/tanks/tanks.dm

@@ -4,7 +4,7 @@
 	lefthand_file = 'icons/mob/inhands/equipment/tanks_lefthand.dmi'
 	righthand_file = 'icons/mob/inhands/equipment/tanks_righthand.dmi'
 	flags_1 = CONDUCT_1
-	slot_flags = SLOT_BACK
+	slot_flags = ITEM_SLOT_BACK
 	hitsound = 'sound/weapons/smash.ogg'
 	pressure_resistance = ONE_ATMOSPHERE * 5
 	force = 5

code/game/objects/items/tanks/watertank.dm

@@ -6,7 +6,7 @@
 	icon_state = "waterbackpack"
 	item_state = "waterbackpack"
 	w_class = WEIGHT_CLASS_BULKY
-	slot_flags = SLOT_BACK
+	slot_flags = ITEM_SLOT_BACK
 	slowdown = 1
 	actions_types = list(/datum/action/item_action/toggle_mister)
 	max_integrity = 200
@@ -60,7 +60,7 @@
 
 /obj/item/watertank/equipped(mob/user, slot)
 	..()
-	if(slot != slot_back)
+	if(slot != SLOT_BACK)
 		remove_noz()
 
 /obj/item/watertank/proc/remove_noz()
@@ -344,7 +344,7 @@
 	icon_state = "waterbackpackatmos"
 	item_state = "waterbackpackatmos"
 	w_class = WEIGHT_CLASS_BULKY
-	slot_flags = SLOT_BACK
+	slot_flags = ITEM_SLOT_BACK
 	slowdown = 1
 	actions_types = list(/datum/action/item_action/activate_injector)
 
@@ -361,14 +361,14 @@
 	toggle_injection()
 
 /obj/item/reagent_containers/chemtank/item_action_slot_check(slot, mob/user)
-	if(slot == slot_back)
+	if(slot == SLOT_BACK)
 		return 1
 
 /obj/item/reagent_containers/chemtank/proc/toggle_injection()
 	var/mob/living/carbon/human/user = usr
 	if(!istype(user))
 		return
-	if (user.get_item_by_slot(slot_back) != src)
+	if (user.get_item_by_slot(SLOT_BACK) != src)
 		to_chat(user, "<span class='warning'>The chemtank needs to be on your back before you can activate it!</span>")
 		return
 	if(on)