Actually, we’ve only gone during the day, but there are some municipal pools open at night. We’ve discovered the Parisian pool system. There are three to four pools withing walking distance from our house, and many others that we could access with a short ride on the metro.
We bought a card for ten entries for 21.50€, and we can go to any of the pools in Paris with it. We’ve gone three times so far and have (I hope) discovered the hours which are less busy. It makes sense really, before work and during lunch are very busy. It is easier to find a place to swim mid morning and mid afternoon. Because we have flexible work schedules we can go during these times and free up space during the busier times for regular swimmers.