Withholding a tenants deposit due to mold?

[Abi Horton]

I just bought a property that had tenements that were older and lived in the house for about 10 years. It came out during the inspection that there was a sewer line blockage and the tenants confirmed there had been “back ups”.

When viewing the property I could visibly see there is mold underneath the linoleum all over the bathroom floor. Some parts black near the toilet. But it’s on a concrete slab so was told I could just change the floor and treat it and be good. 

When the tenants moved out they found a number of spots behind furniture that had quite a bit of mold. After getting it inspected there are 6 areas where the drywall will have to be ripped out and replaced including two walls in the bathroom and I’m looking at $5k+ in remediation. 

Can I withhold the tenants deposit for this for negligence if not telling the landlord there was back ups/toilet problems?