Red dots appearing on river rock

Red Spot Algae. You can take rock out and scrub off. but do not know why it appears.

I had a problem with this, and after much research I learned that it is a freshwater form of coralline algae. I had it appearing on my aquarium glass, always forming the red spots that grow when left alone. A razor blade was required to remove it from the glass. It is connected to an excess of calcium and phosphates, if I remember correctly.

I had a spot of this on my glass (one ring...one tank...over 17 years). I do not have phosphates, but definitely I have calcium.

I found this online that ”seems” informed - Article

djr955 said:

I found this online that ”seems” informed - Article

Click to expand...

Wish I'd seen that 10 years ago! It suggests high iron levels as being part of the cause, and that would explain why it grows out of control in my tanks. My well water is very high in iron.

Thank you for finding that! Even an old guy like me can still learn something!

Lol, google is your friend!

Agree, I have a lot of iron as well. Google is your friend if you vet the qualifications of the author...

As I said

djr955 said:

I found this online that ”seems” informed - Article

Click to expand...

Emphasis on “seems”

djr955 said:

As I said

Emphasis on “seems”

Click to expand...

It does seem fine other than I have never seen it on plants.


I have had this stuff completely take over tanks to the point that the only way to get rid of it was to get rid of the tanks.
If it gets on your heaters, spray bars etc, it's not coming off.
My advice is to get rid of those rocks and anything else that it appears on.
The only thing that I have found that will kill it off is to leave it outside over a Canadian winter (the rocks).
I have a 210 gal tank that I completely stripped down and cleaned, scraped etc at least 5 years ago and this stuff is just starting to re appear in that tank now.
I spent many hours with a wire brush drill attachment, cleaning off rocks. Don't waste your time. Get rid.

Thanks for the replies, and the article. I haven't gone through all the "under" rocks to see if it's spreading in non-visible areas. Hoping I don't have to pull out most of my rocks to try and scrape it all off. And more importantly is correcting the root of the issue. I'll have to look into some more test kits and readings of other stuff. Thanks again.