New member
So I've had green water for around 2 weeks now. Thought it would come and then go with time, beginning to think not.

I've been told the green water is phytoplankton, I did a large water change last week, it cleared up for a few days, but then came back.

I've shortened the light cycle, decreased feeding, run carbon gfo etc.

I've done some reading, could there be a phyto bloom because the food chain is unbalanced, the tanks only 3 1/2 months old, so there's probably nothing to eat the phyto, like rotifers or Copepods?

Could adding live rotifers help clear the water and also produce a nutritious food for my fish?

Any help would really be appreciated?


ReefKeeping Mag staff
Premium Member
Interesting thought. Not sure the rotifers wuld like the brand of phyto you are growing but they might.


New member
Is there something else I should do to get rid of the green water, or is the best cure to just stay patient and wait it out?

I cant find much info on this subject, when I search anthing related to phyto its always the subject of dosing phyto or feeding phyto.


New member
Would adding a bag of purigen possibly help this situation. Here is a picture of the tank.

180 Gallon.

Parameters Read.

Ammonia 0
Nitrite 0
Nitrate 0 (API) Not sure how accurate.
Phosphate 0.06 (With Hanna Checker)
SG 34.5
Temp 76.7 keeping between 76 & 77
PH 7.94


New member
Rotifers will most likely eat it. Another option is UV.

Removing all light for a couple days will do it as well.

With all options, be sure to remove the nutrients causing the bloom.


ReefKeeping Mag staff
Premium Member
I agree lower nutrients.Less input via foods or decaying material in the tank. Skimming and gac and or a resin like purigen can help as well.

A UV will clear it up if yu have one available.


New member

I've added 2 100g bags of purigen to the sump, they fit in between the baffles. I'm already running GFO & GAC, I'm going to change out the carbon today also.

Also added some biospira, figured it could help.

Also took 15lbs of live rock from my established tank, and added it to this tank.

Hoping things will start to clear up, may try to do another water change on Monday.

It is definitely clearer when the lights first come on, and then gets greener during the day. I may try lights out for a day or 2.


New member
What salt mix are you using? I grow Phyto and have found that it does not like to grow when I use reef crystals. Just something else to consider. Additionally it cannot grow with out a nutrient source. Find that source and you should be well on your way to curing your problem.


New member
I used red sea coral pro salt.

Started dosing vinegar on Monday, and finally got the skimmer dialed in how I want it.

Absolutely zero difference.

So to sum up, I'm running carbo and gfo in separate reactors, carbon dosing, cut my light cycle to 8 hours, 200g purigen in the sump, skimming efficiently, feeding 2 cubes every 3 days, turned my fuge light off, mixing water to do another water change tomorrow.- And still have cloudy green water.

So, it's last resort, bought a UV unit, and will install when it arrives next week.

What a PITA this has been.


New member
Just wanted to update you all.

Been a lot of progress, but now, my tank is completely clear, its amazing.

It all started to clear up last Sunday, and day by day this weak its got clearer and clearer. The strange thing is, I only turned on the UV on Wednesday, and by that point, it was already 98% clear.

I started dosing vinegar on the 28th October, following the typical guidelines. I'm thinking that the Phyto bloom was caused by extra nitrates, and I think by Sunday, the vinegar was reducing them and thats what cleared up the phyto. I went ahead and installed the UV anyway, just as an added precaution, and could be a few other benefits along the way.

Here is a few pictures of the progress.


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ReefKeeping Mag staff
Premium Member
Invisible water is nice. Careful vinegar and/or vodka dosing will help it stay that way,ime. Good luck.


New member
I was really puzzling over this until the last picture. I've actually never heard of a phytoplankton bloom in a reef tank with a functioning skimmer before. In fact, I actually add a boatload of phytoplankton every night to my reef tanks, and even with the skimmer off, the live rock clears the tank within 30 minutes or so.

The last picture shows what looks like sunlight coming through a window and illuminating the tank. You will probably want to avoid direct sunlight shining on your tank. It's anecdotal, but a lot of us correlate the high intensity yellow light from the sun (or from artificial sources) with algae growth.


ReefKeeping Mag staff
Premium Member
I know some folks say from anecdotal observations that sunlight encourages phyto blooms; some disagree.
My display tanks are drenched in sunlight from a skylight and bay window and do not grow a lot of phyto for example.A friend of mine whose aquarium has been tank of the month twice strongly favors keeping his tank in sunlight.

Beyond anecdotes ,there have been studies that show phyto in the oceans grows most in the 20 to 40 meter depths not at the surface where more unfilltered sunlight is available .

The thread linked below started as a discussion with Randy Farley regarding the best placement relative to light availability for sponges of various types ; on page two it evolved into a discussion on phytoplankton availability:

This is from an article about phytoplankton growth at variable depths in Puget sound which is fun to read:

This is from it :

"If... we collect phytoplankton at different depths, Then... there will be more plankton at the surface Because there is more sunlight at the surface and phytoplankton need sunlight to live."

and this:

"We think we disproved out prediction because we didn't realize plankton lived so deep... "

This is a stogier albeit more rigorous study on it:

This is from it:

Phytoplankton densities in the surface waters (0-m layer) was nominal compared to the
other 3 strata/layers (20; 40 and 60-m). Phytoplankton densities increased with depth. Abundant
concentrations of phytoplankton coincided with the fluorescence maxima and maximum
concentrations of nutrients.



New member
Interesting stuff.

To note though, I had the window shades closed super tight, so when the tank went green there was hardly any sunlight entering the tank.

Friday and Saturday I actually opened up the blinds and let sunlight shine directly into the tank, it only got direct sunlight for about 3 hours. Anyway, it didn't make a difference to the water clarity, still clear.

So who knows what caused it, but so far it is staying under control.