The fix there is going to be a steam cleaning followed by steel brush with some TSP or oxalic acid (Bar Keeper's Friend) and digging out the top layer of the grout with an oscillating tool.There was no need to replace the tile around the fireplace to deal with that stain.

I use RTAs from Kitchen Cabinet Kings.Flush hot water heater between tenantsUse drain enzyme to clean out drains between tenants to clean old plumbing lines.Use a powdered boric acid based bug killer before tenants move in to keep bugs in check.Use stainless steel and ceramic for sinks.  

I've tried the boric acid,borax, foggers,sprays, even professionals have come twice but I fear they will come back.  

Many sewer services have cameras that can show you the problem and what type of system you have.Assuming you are on City sewer, after you have the lines cleared, treat the plumbing with an acid treatment--I use CLEAN SHOT--4x per year. 

The acid down the field lines was smoking and gurgling , I put down abour 20 gallons and let it sit . 

And am familiar with the income and acid papers I need to provide with her information.

A wash of dilute muriatic acid--USING PROPER PROTECTIVE GEAR AND PLENTY OF VENTILATION.

My name is John Browne and I am a partner Roof-A-Cide.

Nicely acid stained is 6 to 10 per foot so I would not go there.

During that time, she applied boric acid and DE on the tunnels daily, and around the house weekly.