Mobs other than Guardians spawning in cleared Ocean Monument area

After spending a lot of time clearing out an Ocean Monument to build a Guardian farm, we discovered that Guardians are NOT spawning in the farm (in tanks of water) but other mobs (ex. Skeletons and Spiders) are spawning.  I had read somewhere that only Guardians should spawn within the boundaries of the Ocean Monument.  Can someone please look into this?