There are a number of reasons why immersing in a bathtub or swimming pool does not work as a kosher mikveh that can enable a woman to exit niddah status (at least not without major adjustments).
Here are a few of the reasons:
- A mikveh must contain forty se’ah of water, 332 liters at absolute minimum (though mikva’ot typically follow more stringent views requiring more). A woman needs to be able to completely submerge her entire body at once in the mikveh, with the mikveh retaining the minimum volume even if water splashes out. This is well beyond the capacity of bathtubs and of many indoor home jacuzzis.
- A keli, or vessel, cannot serve as a mikveh. A claw footed bath or above ground pool are definitely considered kelim that cannot work as mikva’ot, regardless of the material of which they’re made. When a bath, jacuzzi, or pool is built into the floor of a building, there is debate as to whether it is considered a keli, depending on a number of specific factors concerning its construction.
- A mikveh cannot have any major leaks below water level. According to some opinions, even very small leaks can disqualify a mikveh, though this may depend on their location and whether they are recognizable.
- A filtration system needs to be made in a way that does not serve as a receptacle (beit kibul) for water. If water is taken out and returned, the pipes need to be embedded in the pool. Whether there is a beit kibul for water can also be of concern with water jets in a jaccuzi.
- When municipal tap water systems include tanks, pumps, or even water meters, their water is considered by many authorities mayim she’uvim, drawn water, which cannot serve as the basis for a mikveh.
- A mikveh must have less than a liter (3 lugin) of mayim she’uvim inside it before it first fills up with 40 se’ah of water. So even an outdoor, in-ground pool would have to be emptied before any possible use as a mikveh, and then filled in a halachically acceptable way with rain water (i.e., from the sky or a direct pipe from the roof) or possibly with blocks of ice allowed to melt naturally into the mikveh.
An expert in mikva’ot would have to evaluate and oversee any of the above.