Let’s talk about the difference between fresh and saltwater algae scrubbers.

They are different designs because the algal growth you get in each are different—so the layout of the scrubbers needs to be able to handle this. Freshwater growth tends to be mostly Spirogyra or Cladophora species, which are very long (one meter) and very thin, like a string or thread.

pond algae

This super-long growth means two things: First, it’s going to try to flow with the water flow, which can be a very long pathway. And second, if the growth lets go, there is going to be long strings of growth finding their way into aquarium plumbing, pumps, etc. Since many freshwater people put the scrubber in their display (because they don’t have a sump) this long growth can get all over.

A plus for freshwater is that many of the fish that people keep (guppies, goldfish, plecos, etc.) love to eat this algae, so it can be an endless source of fresh free food. As a matter of fact, it’s relatively easy to have a freshwater tank that needs no outside food or water changes at all, and yet still has low nitrate. And the fish get to eat what they love the most.

Long freshwater growth is fine if your scrubber and installation are set up for it. Generally, you don’t want a waterfall style for freshwater because the long growth flows right down the drain with the water flow—and can extend for a meter past that!

freshwater algae scrubber

If it’s a waterfall in a sump, this growth will get caught up (or will clog) the return pump. And you can’t really put a waterfall over a display, so it must go in the sump. Thus waterfall designs are not recommended for freshwater.

Upflow designs however are particularly suited to freshwater:

  • Upflows are easy to DIY by using a common air pump which many people already use for freshwater.
  • Freshwater people usually want bubbles.
  • Many freshwater tanks have no sumps, which is no problem for upflows because upflows can go into the display.
  • Since filter units in freshwater usually go in the display, the fish can pull the algae out and eat it.
  • The long growth is circulated around (and held) inside the upflow container, keeping most of the growth from clogging other filters or pumps.

freshwater algae scrubber

Now let’s look at saltwater.

The big difference with growth in saltwater is that it grows much coarser and thicker strands (Ulva species, etc.) which tend to be curly but also much shorter than freshwater. So you no longer have to deal with long growth that grows out of the filter. This means that any version of a scrubber will work: upflow, waterfall, or horizontal river. But with saltwater you do have to deal with thick growth that blocks light.

The concept of blocked light is important because algae will die when light blockage occurs. Dead algae cannot hold on, so it washes away, and there goes your filter. It’s ironic: Thicker growth seems great, but it blocks light, which kills the growth. Dead growth is brown like hay and can give water a yellowish look.

So what should you do?

Dead Algae Scrubber

Fortunately the growth does not die right away. It takes a few days once the light has been blocked. So if you clean/harvest before the “roots” die and let go, you are good (algae do not really have roots but do have holdfasts which grab solid surfaces).

There are some tricks to getting more growth before die-off occurs. You might think you can simply reduce the hours of light so it does not grow thick so quickly, but this reduces filtering and thus defeats the purpose, not to mention it give more hours of darkness which does not help the roots. So the idea is to grow fast but not shade the roots too soon.

Here are some things to look for or try:

A waterfall or upflow screen should be illuminated on both sides so the roots that are grabbing the screen get light from both sides. This will let the growth go several more days before the roots get zero light.

Adding strings to a screen or a solid surface lets the growth grow away from the screen or surface, thus getting the roots out of the darkness. Strings by nature are already 2-sided.

strings algae scrubber

Usually the middle of a screen is where growth is the thickest and die-off occurs first. By using very strong light in the middle of the screen, this area can be kept un-grown until an outer ring grows first, thus keeping the middle alive longer. The term for this is “photoinhibition.”

growth rings algae scrubber

By letting your animals eat some of the growth, the time for the growth to get too thick will be lengthened. It's possible the growth will never get too thick with this approach.

eating algae scrubber

Instead of letting the growth go a full 7 to 14 days before cleaning, you can clean half of it every 3 to 7 days. Since you are cleaning / brushing / scraping right down the middle of the growth surface, this opens the middle to more light.

A more elegant way of doing alternate cleanings is to divide the growth surface in half, and clean each at alternate times. A waterfall screen is easily cut vertically for this.

The “most” elegant way of achieving alternate cleanings is to have multiple scrubber units. This achieves the same growth time for each growth area as above, but you do not have to deal with taking/leaving part of the growth behind, and you also do not tear growth in half which can put little bits of growth into the water. It also is the most expensive way.

HOG2x Freshwater Algae Scrubber Santa Monica Filtration HOG2x Freshwater Hang-On-Glass Upflow Algae Scrubber

Remember, the above information is for after a scrubber has been growing, or after you get through a slime stage. No scrubber will look or perform like this when brand new. For an example of a modern freshwater scrubber, here is a high power HOG2x with extra light for high nitrate water.

Happy Scrubbing!