Hello: Is there a danger of sewer gas coming out when I have the toilet removed at the base? (I removed the toilet to replace the flooring. Which should take a couple of days.) Thanks!

In one of our upstairs bathrooms when the water is used for a bath/shower there is a fishy smell in the bathroom and the hallway outside the bathroom. The smell doesn't occur in our other bathrooms or where water is used. There is a plumbing vent that is on the other side of the bathroom wall out through the attic. We only notice the smell in the evening when the water is being used. We are also on a septic tank, which has never been pumped out and the house is around 18 years old. We have only been in the house since May 2000. The pipes our copper. I'm not sure if all the information is needed, but I thought I would cover all possible aspects.