would be something like this I think
tag @e[type=minecraft:armor_stand,distance=..1,tag=LIQUID_TANK,scores={water_level=..1}] add NEARBY_LIQUID_TANK
execute positioned ~-1 ~ ~-1 align xyz as @e[type=minecraft:armor_stand,tag=NEARBY_LIQUID_TANK,sort=random,limit=1,dx=2,dy=0,dz=2] run function remove_water_level
tag @e[type=minecraft:armor_stand] remove NEARBY_LIQUID_TANK
execute if score SUCCESS OBJECTIVE matches 1 run function convert_concrete
(edited)