I am going to sieze (tnks!) the opportunity to write about a mikvaos sugya that i discussed in my sefer mayim rabim (pg. 172).
The gemara says that the din of נתן סאה ונטל סאה only works until rov, but once the majority of the water has been transformed the mikva would no longer be valid. Both Rashi and Tosafos (as well as the Rishonim on the mishnayos) understand this halacha to be speaking about wine and liquids other than water that would fall into the mikva. So long as they are falling into a 40 seah mikva, even if some water is removed afterwards the mikva would still be valid because the wine would be batul until most of the water has been replaced with wine (at which point the wine couldn't be batul to the water). These rishonim would understand that if instead of wine the same system was used to add drawn water to the mikva, it could go on forever and wouldn't be invalidated when the majority of the water is drawn water. The rationale for this distinction is a concept of zeri'ah or hashaka where we consider any drawn water added to the mikva to become purified in the mikva as if it were actual mikva water. However, the Rambam (Mikvaos 4:7) understands that even by drawn water the mikva would only be kasher so long as the majority of water is rain water, not drawn water. The Rosh indicates that the peshat in the Rambam would be that a mikva under all circumstances must retain a majority of rain water, any less would invalidate the mikva presumably on a Torah level.
The Beis Yosef (y.d. 201) explains that essentially the Rambam agrees with the other rishonim that drawn water becomes mikva water upon contact, technically even if the majority of water originates as drawn water. The Rambam holds that when one is manually drawing water and adding water, it looks to the eye that the mikva water has been taken out and will lead to one using a mikva containing only drawn water, therefore the mikva is passul m'drabonon.
Many mikvaos use standard pool filters which draw water from the mikva, filter them through a beis kibbul and pump the water back into the mikva. This is basically the same as נתן סאה ונטל סאה because after a few days or weeks the majority of water in the mikva will end up being drawn water and passul according to the Rambam. Although Shulchan Aruch paskens like the other Rishonim, the Shach is machmir for the Rambam and it is therefore preferable not to use a mikva that has a pool filter. Rav Moshe Feinstein (igros moshe y.d. 1:119) raises an interesting question - according to the beis yosef approach in the rambam that it is a ma'aras ayin issue, it would only make sense if the water is added and removed immediately. If there is a time that elapses between the adding of water and the removal of water, such as separate days, it won't be obvious to anyone that the extra water in the mikva is drawn water. Based on this approach, it would seem that even if the majority of water would be drawn, so long as it isn't done immediately, it wouldn't present a problem. Nonetheless, this heter wouldn't seem to apply to pool filters because when one sees the method that is used to draw and add water, it becomes obvious that after hours, days or weeks of running, all the mikva water will eventually be replaced with drawn water and would certainly be a ma'aras ayin issue. But, the chazon ish (mikvaos tanina 4:10) holds that one can follow the shulchan aruch and doesn't need to worry about the shach and Rambam since according to beis yosef it is at worst a p'sul d'rabonon.