Grey snot algae identified from aquarist's tank sample

Tom Barr

Staff member
Jan 23, 2005
I got a sample of some "grey snot" algae finally and took a look under the scope:

And the species is?

Good old Didymosphenia geminata (Lyngbye) M. Schmidt
A nasty diatom filamentous grey to white snotty thing.

issg Database: Ecology of Didymosphenia geminata

This looks like the bugger.

I think Cavan Allen had the first pesky run in with this species and had a heck of time trying to beat it, thinkng it might be bacteria or BGA.

Turns out it's a diatom from hell.

Anyways, now we know what it is and that it's pretty nasty.

Tom Barr


Lifetime Members
Lifetime Member
Jul 8, 2008

Is this the algae you saw? (see attached pictures) Because I'm struggling with this snotty brown algae, but it doesn't feel slimy, but slightly course. It disintegrates very easily, almost like a spider gossamer or cotton candy. It grows like crazy on my hornwort, which is at the water surface right underneath the lights.

I can easily pull the hornwort out, wash it off, and return it to the tank, but it just grows right back. It grows fast in my other tank.

If this is the same algae, and if you've found a way to combat it, please let me know.

My tank stats:

10 gallon tank.

Anubias Nana
Anubias Barteri
Anubias Nana Petite
Java Moss
Java Fern
Hornwort (used as an excess nutrient sponge to combat algae, not attract it!)

I use Seachem's full line of ferts including, Iron, Phosphorus, Nitrogen, Trace, Excel, Flourish and Potassium. I follow Seachem's recommended dosing amount and dosing schedule. Substrate is typical gravel. I follow the EI regiment and do a 40% water change / vacuum gravel once a week to reset the tank.

I do not use CO2, hence the reason why I use Excel instead. (CO2 is rocket science/voodoo to me).

Nitrates: Typically between 10-20 ppm, depending on whether the test was done before or after the water change.

Lighting: 20 watts total; 2 -10 watt 6500K CFL’s on for 8 hours a day.

The otocinclus don’t touch the stuff, nor do the amano shrimp, and neither does the nerite snail. In fact, I even put the snail on top the algae-infested hornwort, and in a few minutes, it just hops right off. I’m thinking that none of my clean up crew touch the stuff because the hornwort is floating at the surface? But if that were true, then the java fern and java moss should get a good cleaning at least.

I’ve had this tank set up for many months before this algae started rearing its ugly head. Any thoughts? Thanks in advance for your help.