On erev shabbos, or anytime, is it acceptable to use a swimming pool as a mikvah?
It is preferable to immerse in a rabbinically approved mikva; however, when such a mikva is not available, or under extenuating circumstances, is it acceptable to use a swimming pool as a mikvah.
This answer is specifically for men, for whom immersion is only a “custom.” Women must always immerse only in a rabbinically approved mikva.
Sources: See Rash (Hilchos Mikvaos 12), Shulchan Aruch and Rama (YD 201:48); even Noda Biyehuda, who is the strictest concerning this issue, concedes that where the piping is attached to the house &/or the ground the pool is a kosher mikvah, because it is made to be attached in this way (see Iggros Moshe, vol. 4, YD 17, concerning swimming pools). Only water that is drawn via a vessel, such as a pail or tank, would render the pool invalid. However, even water drawn through pipes can be sheuvim, and several poskim have noted the filter attached to a swimming pool can be probelmatic in causing the water to be sheuvim (see Iggros Moshe, vol. 1, YD 110).
Yet, even if the water is drawn (she’uvim), many authorities write that the pool is acceptable for tevilas keri. For the “extra kedusha” of Erev Shabbos there is some debate over whether it is suitable. See Shaarei Mikvaos (29:10), and see Chelkas Yaakov (118) who writes that the mikvah is valid for tevilas keri, an opinion also found in Shevet Halevi (4:117); Shraga Hameir (vol. 4, 44); Torah Lishmah (375). However, the filter might cause the water to be zochalin, which some poskim consider unsuitable even for tevilas Ezra (see Zichron Yehuda 2:61; Machazeh Avraham 1:13; Be’er Moshe 5:21; . Minchas Yitzchak 3:64; Shraga Hameir 3:57; Vayaan Yosef vol. 1, 46:2). According to Iggros Moshe (ibid), the filter does not cause a problem of zochalim.