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Narrowing Down Cause of Green Water


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So out of all my tanks, I have one tank that tends to keep green water.  I have been struggling with this tank now having green water for six years.  I tried to reset this 35g hex tank two years ago by removing all the substrate (sand and dirt) and replaced it with ecocomplete (3" depth.  Plants in it are:  1- aquarium dwarf lily, 2- amazon swords, a full thick carpet of dwarf sag, a row of ijungle val in the very back behind the amazon swords, and frogbit that covers roughly 1/4 of the top.  A few months ago I discontinued using root tabs. In regards to liquid fertilizers, A while back I was using dry ferts mixed but switched over to easy green liquid fertilizer.  I have for the last month discontinued even easy green.  Current nitrate levels are beneath 10 ppm.  The aquarium does have 1 Nicrew Gen2 Classic on it for light, with a dimmer bringing it to 75%.   Natural sunlight does get on the tank for roughly an hour each day (7 - 8 am).  The light schedule is 6 am - 10 am., and 6pm to 10 pm (8 hours total).  Like all my tanks I do supplement with passive CO2 (257ml for this one) daily.  

So my fellow fish nerms, I was wondering what is your suggestions for my next step?

Should I test for phosphate or any other nutrient? And if so what tests would you recommend.  Currently I use API liquid master test kit and I use the easy test strips by tetra.  I do also have the API test strips too.   

After six years with this tank, I am at a complete loss on how to stop the green water.  I am considering going back to using a UV sterilizer.  I don't mind a little bit of cloudy water as long as I can see the fish, but having green soup is no fun.  


Edit:  The current filter is the moving bed the Coop sales.  Which btw is working great.  

Edited by Ben_RF
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It is an interesting problem. Substrate doesn't seem matter as sometimes my green water tanks are bare bottomed. The two consistent factors are fertilization (which can be both fish poop or additives) and strong light.


The 3 tanks above have the same pH, ammonia, nitrite and nitrate according to the API Master test kit, even though Nerm get more fertilizer in the way of Root Tabs. All 3 get Easy Green and Easy Iron. Lighting is about 11 hours a day but growing shorter with each day.

The Nerm tank gets the most fertilizer of the three aquariums and the greenest water.

The 1930s tank stayed clear when it didn't get much sunlight.


But once it got full sun all day, it turned green. And 1930s aquarium has inert creek gravel and doesn't get fertilized.


That is clear as mud, right? One tank in the dirted tank project is green because is gets more fertilizer. Except for the fertilizer, all the tanks are identical, setup at the same time with same plants and are treated the same.

But, the 1930s aquarium has green water, gets no fertilizer and wasn't green until it started get a lot more sunlight.

And finally this aquarium was clear until I put started (over)feeding constantly. Once I cut back on the food, it cleared up.


The problem with green water is you can never grow it when you want it and it grows when you don't want it.

One thing for sure that will clean up green water in a heartbeat is Daphnia. Daphnia will clear water overnight. Cutting back on light clears it up also.

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Hey Ben it sounds like your tank has gone through several changes within a matter of a few months since you've mentioned changing from dry ferts to an all in one down to nothing. In that regard your plants may still be adjusting to the variable doses of macro/micronutrients. I wouldn't necessarily cut off completely dosing fertilizers because the green water can occur from an imbalance of nutrients. What I mean by this is there is a possibility that the plants are being limited by a specific nutrient which prevents your plants from uptaking the rest of your fertilizer.

First thing I would suggest is to have solid regime as far as dosing, water change schedule, and photo period. It can take some time for plants to adjust to being fed different fertilizers just because of the different nutrient concentrations. So if you plan to just use easy green just stick with the recommended dosing for a few months so we can establish a baseline. Its much easier to make 1 small change to a tank when everything else is running the same because small changes in nutrients could take a few weeks before you see the result of that change.

In regards to the natural sunlight, it could be one of the factors for an imbalance of nutrients since you can't decide on duration or intensity of light you get from the sun. I'm not positive how much the direct sunlight is specifically affecting your setup I just wanted to point out that is a variable factor that you may want to look into.

What is your water change schedule like? (do you remove decaying leaves or gravel vac excess organics?)

Also I dont think you really need to grab the phosphate test kit. If you're really set on buying look into buying the api pond test kit. Pond test kit is the same as the master test kit except you get phosphate instead of nitrate. Unless you are doubling or tripling EI dose from dry ferts or going way over the suggested easy green dosing there shouldn't be a need to test for it. Regular water changes every week or two should combat it.


I'll mention that you could use API algaefix which will clear it up for you but I don't recommend it for 2 reasons.

1. Killing all the algae will give you an ammonia spike

2. It will clear up your tank but unless you solve the underlying issue it could always come back

This is all speculative by the way, Im just commenting on what stood out to me in your setup. Hopefully I gave you some other options to look into. Good Luck.


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Thank you all for your responses.  

I did try leaving it the same for over a year, to no success.   But you are right, that with the current changes I should give it a few months atleast to see how they do instead of 30 days to 45 days time frames.

Once again thank you all for your help. 

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