He demanded a storm door, a railing installed on the outside stairs (after installing a railing he complained that it's not made of iron and asked me to redo it, but that's where I drew a line), he called a plumber in several times about silly issues that only happened because he consistently overflowed the bathtub, today he became convinced that all the windows in the house need to be replaced.

@Mike Bryant - Lowes sells a kit for painting bathtubs and tile that I've used in the past.

But my car fits a bathtub (no joke) and a 6 foot ladder in it and pretty much anything I want from home depot.  

If the home only had one pipe that ran to one faucet it would do what you believe it's doing in protecting the water line from freezing, but homes have multiple water destinations, kitchen sink, dishwasher, ice maker, bathtub, vanity faucet, showers, toilets etc...with multiple water line branches.That system works only as a localized system on the trunk line or water line that is in direct line from the place you have it installed, back to the water heater, there are other water lines that branch off from the water heater at some point and odds are greatly not in your favor that you're pulling the water from those lines and recirculating it.  

Speaking with the inspection company the growth is in areas that could have been the tenant being careless. ie: water from shower and bathtub.

Upstairs shower had a bad (Horrible) connection on the pipe leading from the bathtub to the shower head.

The bathtub draining pipe was also not connected water just run out beneath the house.

The liner in the bathtub... so grossthis is on the ceiling..

This is more of a cleaning list, but has a few items that may be helpful to think about:All: Clean window wells with bleachAll: Clean baseboards, window sills, and closet shelves with degreaserAll: Clean doors (inside and out, as needed) with degreaserDrains: snake out between tenantsBathroom: Clean bathtub/shower stall with bleach cleanerBathroom: Clean toilet (all surfaces) with bleachBathroom: Clean toilet bowl with AjaxLaundry/Bathroom/Kitchen: Clean baseboards and where floor meets wall with bleachLaundry/ Bathroom/Kitchen: Sweep floorLaundry/ Bathroom/Kitchen: Mop floorWater heater: check temperatureBathroom/Kitchen: Clean counters with bleach cleanerBathroom: Clean mirrors with WindexBathroom/Kitchen: wipe drawers and medicine cabinet shelves with bleachKitchen: Wash cabinet doors with degreaserAll: Wash walls as needed with bleach or degreaserKitchen: Wipe/scrub fridge inside and out with bleachKitchen: Wipe/scrub stove inside and out with bleachKitchen: If garbage disposal, add vegetable oil to keep it greasedKitchen: Wipe/scrub dishwasher inside and out with bleachLandry: Wash shelves with degreaser or bleachAll carpet: vacuumAll carpet: steam clean carpetOutside: Clean doors if neededOutside: Clean walls near entrance and steps, if neededOutside: Prune shrubsAll: clean all vents and cadet vents

The bathtub drain must have been slow or blocked.