Does It Affect Purity or Prayers?

When I was about 13 or 14 years old, I used to go to our water tank. It was quite large—large enough for a person to drown in—but I would hold onto the metal rods to swim. Because of the chemicals in the water or the physical sensation, I would occasionally experience an ejaculation. At the time, I was very young and didn't understand what was happening, but this occurred two or three times. ​This was the main water tank for our house, which the entire family used for their daily routines. Now, many years later, I remember this and feel it was a sin. I am wondering what is that kaffara for this ? Does my family need to repeat their past prayers, considering four or five families lived in that house and used that water?"