Before getting into the wet vs dry skimming debate, it helps to have a brief review of why we use protein skimmers on reef aquariums. Aquariums are closed aquatic systems. There is a constant input of organic substances from fish and coral food. All of the marine life, from bacteria to corals, constantly release organics in the form of oils, slime, solid waste and cellular fragments. These organics can be complex compounds dissolved in the water or tiny particles floating around the tank. In the ocean these substances are continuously washed away by tides and carried into deeper water, or recycled for use in the food chain. Even with partial water changes our reef tanks tend to accumulate organics. Some of these substances eventually break down and end up as ammonia, nitrate, phosphate and sludge. So how can you remove these organics from the system? With a protein skimmer.

How Protein Skimmers Work

How a protein skimmer works diagram

Protein skimmers are based on a filtration technology called foam fractionation. Certain organic substances can be removed from saltwater by whipping them into a frothy foam. Foam fractionation works by injecting tiny bubbles into a column of water. Organics attach to the rising bubbles and form a concentrated foam at the top of the skimmer column. The foam is less dense than the water and sits above the surface of the water. Aquarium protein skimmers use specially designed water pumps that agitate the water to create a constant stream of microscopic air bubbles. The pump injects water and air bubbles into the protein skimmer. The dissolved organics and tiny particles grab onto the air bubbles as they rise to the top of skimmer column and form foam.  The foamy organic waste is “skimmed off” and trapped in a collection cup.

It’s all about the foam

whether wet or dry skimming, it is all about the foam

The air bubbles, coated with organics, rise to the top of the skimmer.  As the bubbles come together at the top, they form a wet foam just above the water surface. Protein skimmers are designed to channel the foam farther up the neck of the skimmer. As the foam rises, excess water drains away, creating dryer foam. This foam, which is a combination of water, dissolved organics and organic particles, is what is collected in the collection cup.

Wet vs Dry Foam Skimming

AquaMaxx NF-1 Nano Protein Skimmer - In-Sump

By making adjustments to the skimmer, you can collect foam ranging from very wet to dry, fluffy foam. There is no standard definition to what wet or dry foam is. Dry foam has had a chance to drain off most of the water, leaving a light, fluffy foam. Wet foam carries a lot of water because it has not drained very long. So, what’s the difference?

Wet foam

  • Really wet foam adds excess tank water to the cup. You’ll have to replace the lost water with fresh saltwater.
  • The collection cup or jug will need to be emptied often.
  • The skimmate may be full of particles, like algae, and form a cake of organic debris in the cup.

Dry foam

  • The foam contains less water and the cup won’t need to be emptied as much.
  • Dry foam loses some of the organics as they drain away in the water.
  • You won’t have to top off as much.

Which type of foam is best?

Reef aquarists may debate the topic but the science is in. The most efficient way of removing organics with a protein skimmer is by collecting a slightly wet foam. If you want to maximize removal of organics, adjust the skimmer to produce foam so that the foam contains some water but isn’t filling up the collection cup every few days. This will let the skimmer remove the organics in the foam and the concentrated organics riding along in the water. If the foam is consistently light and fluffy, make an adjustment to the skimmer. There is nothing wrong with running a wet foam. If you’ve got a big collection system it’s almost like making a continuous water change. If you top off with seawater the salinity should remain stable. Wet foam tends to remove a lot of particulates, foaming a thick sludge layer in the cup. Some aquarists like this. Others would rather have a filter bag or fleece roller handle the particulates and let the skimmer focus on dissolved organics.

Final thoughts

The good news is you can’t hurt your tank by skimming with wet or dry foam. For maximum organics removal, set your skimmer so it produces foam with a little water. You can experiment to find the right balance for your reef tank. If you want to skim wet foam, be prepared to empty the collection system often and replace lost water with fresh saltwater.