100 Litre Aquarium Skimmer Explained: A UK Buyer's Guide

Short answer: a 100 litre aquarium skimmer is a surface skimmer sized for roughly 100 to 150 litre tanks. It continuously draws the top layer of water through a comb to clear the oily film that blocks gas exchange, so more oxygen gets in and light gets through. Before you buy one, though, it is worth checking that the film is not a sign of a wider water-quality problem.

When you have just set up your first tank and an oily film appears on the surface after feeding, and you notice the fish spending more time near the top, it is natural to worry about oxygen and water quality. This guide explains what a 100 litre aquarium skimmer actually does, how to tell a cosmetic film from a real problem, and how to fit one safely.

What is a 100 litre aquarium skimmer and what does it do?

The surface of a tank is where oxygen enters the water and carbon dioxide leaves. When a greasy-looking film builds up there (from oils, uneaten food and other organics), it acts like a lid: gas exchange drops and less light reaches your plants. A surface skimmer pulls that top layer of water in through a floating comb, traps the film in a sponge, and returns cleaner water, so the surface stays bright and the tank breathes. It is a different tool from a protein skimmer, which we cover in our aquarium protein skimmer guide.

Oily film and fish at the surface: what is actually going on?

First, do not panic and buy the first thing you find. The two things new keepers notice, an oily film and fish spending time near the top, can have different causes, so work through them in order.

1. Check the water first. Test ammonia, nitrite and nitrate, and check the temperature. Fish gasping at the surface can be a sign of low oxygen, but it can also point to an ammonia spike in a tank that is still cycling, so rule those out before anything else.
2. Look at feeding. Overfeeding is the most common cause of a surface film, because uneaten food and oils rise to the top. Feeding a little less often clears many films on its own.
3. Then consider the surface. If the water tests are fine and the film keeps returning, that is when a surface skimmer earns its place, clearing the film continuously and improving oxygen exchange.

Surface skimmer, filter, air pump or protein skimmer?

It is easy to get lost when a surface skimmer, a filter, an air pump and a protein skimmer all appear in the same search. Here is the plain version: your filter cleans the bulk of the water, an air pump adds oxygen by agitating the surface, a surface skimmer removes the film from the very top, and a protein skimmer strips dissolved organics (and is really a marine tool). For most first tanks with a stubborn film, the surface skimmer is the piece that is missing. If you want to fine-tune how hard it pulls, our adjustable aquarium skimmer guide goes further.

How to fit a surface skimmer in a 100 litre tank

Fitting one is simpler than it looks, and getting the water line right is the whole trick. Knowing how to fit a surface skimmer in a 100 litre tank before it arrives takes the anxiety out of set-up.

1. Mount it on the glass with the suction cups, in a spot with reasonable flow past it.
2. Set the height using the 3-level bracket so the floating comb sits right at the water line.
3. Let the comb float so it tracks small changes in level as water evaporates.
4. Start on the lowest flow and turn it up only as needed, which keeps the intake gentle so it will not harm small fish or fry.

Choosing the AquarSurfa Pro Clear for a 100 litre tank

For a first tank in the 100 to 150 litre range, the AquarSurfa Pro Clear surface skimmer covers the basics without fuss. It uses a floating comb that self-adjusts to the water level on a 3-level height bracket, an adjustable flow dial so you can start gently, and a built-in air-intake silencer to cut gurgling. A corrosion-resistant stainless-steel shaft suits both fresh and salt water, heavy-duty suction cups hold it steady on the glass, and it comes with an internal sponge, needle cleaning brushes and a 1.5m cable. It is £75.90 inc. VAT (£63.25 ex. VAT), ships from UK stock with free next-day dispatch, and comes with a 12-month UK warranty and 30-day returns. It is rated 4.7 out of 5 from 341 reviews. Check the floating comb weekly and rinse the sponge every couple of weeks in old tank water; replacement sponges are available.

Frequently asked questions

Do I need a surface skimmer for a 100 litre aquarium?

Not always. If an oily film keeps returning after you have checked your water parameters and cut back on feeding, a surface skimmer clears it continuously and improves oxygen exchange. If the film only appeared once or your tank is still cycling, sort the water and feeding first.

Are fish at the surface always a sign of low oxygen?

No. Fish near the surface can indicate low oxygen, often because a surface film is blocking gas exchange, but it can also mean an ammonia or nitrite problem in a new tank, or simply that food is up there. Test the water first, then treat the cause.

How do you fit a surface skimmer in a 100 litre tank?

Mount it on the glass with the suction cups, set the 3-level bracket so the floating comb sits at the water line, and let the comb float so it tracks the level. Start on the lowest flow and increase gradually so the intake stays gentle around small fish.

Is a surface skimmer the same as a filter or an air pump?

No. A filter cleans the bulk of the water and an air pump adds oxygen by agitating the surface, while a surface skimmer removes the oily film from the very top so gas exchange and light improve. Many first tanks already have a filter but are missing surface skimming.