Previously, I used to rub my penis on the bathroom tiles walls, so some amount of semen may get onto it. When I learned that this is haraam, I stopped doing it. Now I want to make my bathroom tiled walls pure once again. So what do I do?
I washed it but then the washed water got into my tub which was kept just below it. What should I do?
First of all you should know that some scholars consider semen (many) as (ritually) clean (tahir) so washing it and wasting water for this propose doesn't make sense at all (see for example islamqa #170012).
Secondly assuming semen is considered as najis (not ritually clean) G. Bach already quoted this hadith
A Bedouin urinated in the mosque. Some of the persons stood up (to reprimand him or to check him from doing so), but the Messenger of Allah (ﷺ) said: Leave him alone; don't interrupt him. He (the narrator) said: And when he had finished, he called for a bucket of water and poured it over. (sahih Muslim)
so the matter of purification is easy! note that certainly people prayed later on this place were that Bedouin urinated so don't take purification to extrems!
Scholars say that a najasa is considered as removed once neither the odor, the taste or color of it is still visible. And a person is only asked to remove najasa as good as he/she can.
Thirdly also note that the last act in wudu (ablution) or (optional) ghusl (washing) is washing the feet, so you won't step into any najasa once you've finished!
And finally on the issue of misusing water you may read my answer on: Is it permissible to re-use the water that has been already used [by myself] for ablution? and read this fatwa islamqa #1285. Note that Allah dislikes people who commit excess:
and eat and drink, but be not excessive. Indeed, He likes not those who commit excess. (7:31)
and described his servants as people who neither spend excessively nor sparingly (25:67).
And have in mind that wudu and ghusl etc. are purification rituals they are meant to make you feel clean or prepare you to pray, they just make a clear cut between what you have done before and they -in first place- don't intend to make you clean from a hygienic point of view rather than purifying your soul, mind etc.
Semen is already pure you don't need to purify it by water or anything else like that.
