Green Mat Algae

Coralogy_ · Jan 22, 2026

Hi! I've been having some issues with my tank and I'm hoping someone can help me identify what I am looking at so that I can resolve the issue.

It all started when my phosphates and nitrates bottomed out near end of December. I lost all my SPS (RTN) in the span of a week and experienced BJD across my hammers and torches. I am still dealing with dying torches and hammers, but softies and other LPS look healthy. Previous to the bottoming out, I was dosing AFR, carbon dosing, dosing amino acids, and had a reef mat and skimmer running. After my phos and nitrates bottomed out, I had what I believed at the time to be dinos. So, I stopped dosing amino acids, let my tank get dirty by pulling the skimmer offline and tested phos and nitrate daily to get them up.

Here are my current parameters:
Salinity – 1.025
Alk – 10.5
Calc – 439
Mg – 1400
Nitrate – 7
Phosphate – 0.14

During this time when my corals were dying, I did an ICP (Triton) test in early January to ensure parameters looked okay. Everything was in line except Iodine was a little low (36.00 µg/l) and Silicon was a little high (283.00 µg/l). I swapped out my RODI filters to reduce Silicon input.

Now, the stuff growing is a thick green mat, and it is growing across sand, rocks and coral skeleton. It sometimes has air bubbles, but not often. Below is a photo of how it looks in the tank. I did get a microscope because I thought it was dinos, but I am not seeing any dinos. When I gather the sample of the green mat, I shake it vigorously in a bottle to try and break it up, but it just stays clumped. The microscope photo is at 500x. Any idea what the green stuff is? Seems to be some sort of green hair algae, but I am not sure. I've also added a video of a sample from my sand, which I'm not really seeing dinos in.

davidm777 · Jan 22, 2026

Hello,
Rookie reefer here but I been watching and reading all I can. I would like to throw some thoughts on what I have heard. Hopefully people who know more can jump in coming from experience.
I heard Dong say AFR contains calcium formade which is a form of carbon dosing that promotes vibrio. Vibrio is a bacteria family that isn't all that great for corals. If you were already dosing carbon on AFR and then adding carbon on top of that... could have been the reason for bottoming nutrients. Now you could be dealing with bacterial imbalance.
Julian Sprung recommends getting 2 white buckets as deep as you can. Fill one with tank water and one with RODI water. If the water from your tank is significantly yellower your biome (not sure if this is the word... bacterial funa is what I mean) is at levels that could be unhealthy. I won't make recommendations but you can look into this.

Coralogy_ · Jan 22, 2026

davidm777 said:
Hello,
Rookie reefer here but I been watching and reading all I can. I would like to throw some thoughts on what I have heard. Hopefully people who know more can jump in coming from experience.
I heard Dong say AFR contains calcium formade which is a form of carbon dosing that promotes vibrio. Vibrio is a bacteria family that isn't all that great for corals. If you were already dosing carbon on AFR and then adding carbon on top of that... could have been the reason for bottoming nutrients. Now you could be dealing with bacterial imbalance.
Julian Sprung recommends getting 2 white buckets as deep as you can. Fill one with tank water and one with RODI water. If the water from your tank is significantly yellower your biome (not sure if this is the word... bacterial funa is what I mean) is at levels that could be unhealthy. I won't make recommendations but you can look into this.

Thanks for the response. I am in full agreement with you. I just so happen to go to the LFS where Salem works, and he told me something similar and recommended activated carbon and swapping it out weekly to reduce my DOCs. I've done two water changes in a white bucket since and can confirm my water is a noticeable yellow tint. I think my solution here might be to continue increasing biodiversity and ride it out. I've been dosing PNS ProBio bacteria and got some bio media from a buddy with a nice established tank. I'll continue doing research on diversifying bacteria. If anyone has any recommendations on getting rid of the green mat, that would be great. Very possible it might just go away with diversification and time.
Overall, lesson learned! Won't be carbon dosing or adding amino acids anymore :)