Oils cause bubble popping.
I discuss it here:
What is Skimming? by Randy Holmes-Farley - Reefkeeping.com
from it:
Bubble Popping
Other critical things can occur at the foam draining stage, and they usually impact skimming negatively. One is the addition of materials that cause bubbles to pop prematurely. Excessive oils, for example, cause this to happen.
When typical oil droplets are added to a reef aquarium, they quickly arrive at the skimmer. A pure oil droplet is largely hydrophobic on all sides. Oil drops work their devilish tricks in skimmers by spanning across the water between two air bubbles in a foam (Figure 4). Once an oil droplet spans the water gap between bubbles, the amphipathic molecules on both of the bubbles' surfaces spread along the interface between the oil and the water (if they were not there already) and connect both of the air gaps with a continuous line of amphipathic molecules along this oil/water interface. Once these amphipathic molecules are in place, the interaction is unstable. The surface tension pulls at the oil drop (Figure 5), and it simply comes apart. The bubble ruptures from the site of the oil drop, and the effect is that the bubbles combine, or pop entirely. The reason that this does not happen in the absence of an oil drop is that to cause a rupture requires the water present between the air bubbles (or between a single bubble and the nearby atmosphere) to become exposed as fresh air/water interface. In fact, it requires a continuous line of water molecules to become exposed all at once.
Because such a rupture would require a large number of hydrogen bonds to be broken simultaneously, it simply requires too much energy to actually take place. When the oil drop is there, the water molecules are no longer exposed, but rather the oil or amphipathic molecules, which are much "happier" to be exposed to air, and the droplet ruptures, breaking the bubbles on either side of it into one larger bubble. That process continues until no foam remains.
Bubble popping can also be caused by hydrophobic solids, although that process is likely less important to aquarists than is popping due to oils.
Bubble Popping in Marine Aquaria
The effects of this bubble popping process, if not the mechanistic details, are easily observed in an aquarium, where many things may cause a bubble popping effect. One cause that most aquarists encounter is oil from their hands. After reaching into a saltwater aquarium, skimming action often comes nearly to a halt as bubble popping dominates foam drainage and collection. The popping will proceed until the oil is somehow removed. Among other ways, oil can be removed by splattering it above the foam height in the skimmer, being foamed out bit by bit, being emulsified into the general foam as very, very tiny droplets which no longer span air bubbles, becoming attached to solid objects and removed, being consumed by tank microorganisms and by eventually dissolving into the bulk tank water. Many foods used by aquarists have a similar effect on skimmer bubbles.
As an aside, the bubble popping action of hydrophobic oils is exactly how most anti-gas medications for humans function. Simethicone is really polydimethylsiloxane, which is a hydrophobic polymer liquid. It pops bubbles in your stomach or intestine, and permits the gas to be eliminated. Antifoaming agents also are the basis for a large number of industrial products that work on the same principle. Other things also cause bubble popping. One of these is the fatty acid supplement Selcon. It causes bubble popping in the same fashion as skin oil droplets. Hydrophobic solid objects can also cause popping. Fine particles of activated carbon, sand, inorganic precipitates, or granular ferric oxide/hydroxide, once coated by organic compounds, can serve to break foams in a manner analogous to the described for liquid oils.
