I live on a 3rd floor condo and there is no condo above me, there are two below me. I've noticed lately that my bathroom sink drains slowly. I put liquid plumber and it temporarily fixed the problem. About a week or two later it was draining slowly again. I left it as is. Today after coming home for lunch I noticed there was a low level of dirty black water in the bathroom sink. Where did it come from? I checked the bathtub and it was fine. I let hot water run in the bathtub for about 30 seconds and it drained fine. I left and when I came back home about 2 hours later, the sink was full of the black dirty water. When I drained it I noticed the water was warm. I used liquid plumber again and it appears to be draining fine now. Where is the water coming from? Will the liquid plumber damage the pipes? This building was built in 1985. There are 2 exact condos below me and I do have a common wall neighbor who's bathroom is at the same spot. Should I call a plumber? I'm sure the problem will be back. Is one of my downstairs neighbors clogging it? I don't have long hair and I don't put anything down my drains other than water and soap. Thank you.

I believe your condo plumbing shares some drain and vent pipes with the neighbor next door. Your black water was their black water a little earlier in the day. If you are part of a home owners association or have a property manager check with them regarding rodding the drains and the vent. The structure outside the drywall surface of your unit is usually not your responsibililty for repair.