SB 6.9.10
dravya-bhuyo-varenapas
turiyam jagrhur malam
tasu budbuda-phenabhyam
drstam tad dharati ksipan
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
And in return for King Indra’s benediction that water would increase the volume of other substances with which it was mixed, water accepted one fourth of the sinful reactions. Therefore there are bubbles and foam in water. When one collects water, these should be avoided.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
If water is mixed with milk, fruit juice or other similar substances, it increases their volume, and no one can understand which has increased. In return for this benediction, water accepted one fourth of Indra’s sinful reactions. These sinful reactions are visible in foam and bubbles. Therefore one should avoid foam and bubbles while collecting drinking water.