Green Tank Water Won't Go Away. Help!!!


New Member
Jun 14, 2017
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Hey all, I'm writing this because I am absolutely desperate for help. I have green tank water, and I have tried everything I can think of to get rid of it, but it just keeps coming back again and again. Are you guys ready, because this is an epic tale with lots of twists and turns and MISTAKES.

Ok so here goes. A bit of background: my dad has kept fish my whole life on and off. He's had everything under the sun, from goldfish, to cichlids, etc. He basically keeps a very natural tank, lots of river rocks, drift wood, and live plants. It seems like it was pretty easy, or at least he didn't have a lot of trouble with it over the years. As a birthday present for my daughter he wanted to set up a tank for her, and I thought, hey, that sounds fun, she loves fish, and how hard could it be (que eye rolls from any half way knowledgable fish keeper).

In January of this year we purchased the tank and got it all set up so it could start cycling before we put the fish in.

Mistake #1: I bought a tank from a local fish store, but having little to no knowledge about fish keeping what I ended up purchasing was a foreign made fish tank. It is an Askoll Pure XL tank, 25 gallons, with a built in rolling filtration system, and LED lights that also include black lights. It has a built in timer which we have set up to run about 8 hours daylight, 4 hours black light, and then off overnight.

I say that this was a mistake for a few reasons. One being that the tank came with all the filter media included. Filter sponges, ceramic filter media, and then two packets which I'm guessing included charcoal, as well as nitrate and phosphate reducing media encased in a microfilter holder. The problem is I have searched and searched and I cannot find a way to get replacements for the charcoal/nitrate/phosphate mixed packets here in the U.S. They're available in the UK, Italy, etc. but no one seems to ship them to the U.S. The tank ran just fine for about 2 months, then the packets were spent and the trouble started.

Mistake #2: We put the tank in my daughters playroom which is one of the brightest rooms in the house as far as sunlight goes. It was getting a full days almost direct sunlight. However the tank has been moved to her room, which is the darkest room in the house, and receives just a very little bit of ambient light during the day.

Mistake #3: I replaced the packets with a Fluval charcoal media bag. I didn't realize at the time that there was also a mix of phosphate/nitrate neutralizers in the packets as well. The plants (fake), rocks, and ornaments in the tank started growing lots of green algae, I'm assuming because our water has high amounts of phosphates and nitrates. I live in the midwest and we are surrounded by farmland, so our ground water is just full of nutrients from fertilizers. However the water was pretty much clear at this point, just actual objects had stuff growing on them.

Mistake #3: since the tank was growing all kinds of green algae, and I am a total newbie to this fish keeping thing and had no idea I did the worst thing possible. I decided to clean all the filter media, all the ornaments, and plants with water right out of the tap. So yes, I completely killed my bio filter and all the good bacteria in the tank. That's when the green water started to appear. I bought some API Algae FX to help get rid of the green water. It worked initially but for about a week, but then it just got too out of control. I also bought my own filter media bag, and some nitrate/phosphate neutralizer media, and charcoal media to make my own packets. However so far it hasn't seem to help or made a difference in getting rid of the green water. It got so bad that you couldn't even see the fish in the tank or anything else for that matter. (pictures attached of it at it’s worst). In the process of all of this we lost 1 algae eater, 4 danos, 2 tetras, and 3 lyre tail mollies, and one dalmatian molly.

I took some of our tank water in to be tested, and the guy at the fish shop said we had normal levels of ammonia, nitrates, etc. He was kind of a jerk though, and didn't even test for phosphates, and wouldn't test the tap water sample I brought in.

Out of desperation I bought a Sun Sun JUP-01 UV sterilizer to get rid of the green water. After running it for a few days we started to see improvement, and it continued to clear until the water was no longer green after about a week and a half. However the water remained very foggy, almost a grey/white fog, which never went away. I took the UV filter out, did a 50% water change, and what do you know, after a few days the water started greening right back up.

This is when we decided to move it upstairs to a different room. I completely emptied the tank, scrubbed down all the gravel (in tank water), ornaments, and plants with tap water. I rinsed the filter media in tank water to try and preserve the bio filter. After moving the tank everything was replaced, and it was refilled. I used SeaChem Stability, Prime, and Clarity to try and kick start the tank, as we had two little tetras still alive to go back in the tank.

It was awesome to have it all lovely, and clean. We let it go for about a week, then we add a few more fish, two more tetras, and 4 danos. After a week and a half I did the first water change, 25% just to clean things up. All I did was gravel vac, leaving everything else alone. Then it started again, the green started to creep back in slowly. After about a week of seeing what would happen I did a maybe 40% water change to try and help clear the green, and added some microfilter sponges to the filter, and it got even worse (last photo). And that’s where we are at now as my harrowing tale comes to an end (so far).

I’m so frustrated, mostly because this has been anything but an enjoyable experience, but also because I cannot seem to find anyone who can help me solve this problem or who is willing to offer constructive advice. And after much trial and error I'm still not able to solve this problem.

The only things I can figure are one or a combination of these factors:
1. The filter on the Askoll tank is total crap and isn’t filtering the water well enough to keep it clean which is allowing a quick nutrient build up, and then not cleaning dead algae when I try to treat.
2. The filter doesn’t work without the handy little packets, which I unfortunately have no way of getting.
3. The lights are just way too bright, so even having them on for 8 hours is too much.
4. Our water is insanely high in phosphates and nitrates so it’s just a giant algae growing machine.
5. There are not live plants in the tank to help consume the nutrients and shade the tank from the lights.

Alright everyone, that’s it. As I said I’m totally desperate and any and all constructive advice would be an amazing help.

Attached are photos of:
- Tank at it's worst green water stage (2 views)
- Brand new tank (before fish).
- Tank after running UV filter for about a week and a half
- Current tank situation
- Ambient light photos to show how much light it gets.


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New Member
May 31, 2010
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I would try reducing the light levels at first, algae can still use blue light to photosynthesise so your 8 hours white light and 4 hours blue is 12 hours of light for the algae to grow under. As a comparison my tanks are planted and I only use 6 hours light a day total.

I have never had a freshwater tank without plants though so I'm no expert but I would definatley reduce light as a first port of call.


Supporting Member
Tank of the Month!
Feb 25, 2009
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Welcome to TFF. :hi:

Green water is due to unicellular algae that do not attach to surfaces but live in the water. It is generally said to be caused by nitrates, phosphates, ammonia/ammonium and in the presence of light. Blackouts usually don't last long-term because the food source is still present when the light returns. They also have a side effect, namely dead algae that can quickly foul the water and in turn be a food source for the unicellular algae.

Water changes, while the "first" treatment for almost anything, will not help here, and will make it worse as you've mentioned. The "food" is being increased with each water change. UV sterilization will deal with this algae (the only one it does handle) and some recommend diatom filters. Before spending more money, I would suggest looking into the source of the "food" and trying to rectify that. Also reducing the light. Sunlight is always bad for an aquarium, but you have now solved this aspect. The tank light can be on for less time. You don't have live plants, so the light can be less intensity than it otherwise might have to be, and the duration is of less importance regarding plants, but fish are impacted significantly by light, both intensity and duration. A timer to keep the light period the same each day is a good investment; they are very cheap, one suitable for a table lamp is all you need. I would set it to be on when you (your daughter) are generally home to enjoy the aquarium. As another member mentioned, any and all light over the tank is pertinent, to fish and algae, so keep that in mind. The fish need a space of total and complete darkness (no ambient room light) each 24 hours.

I also prefer finding the cause and rectifying it whenever possible, as opposed to "band-aiding" with special filters, UV, etc, etc. Keeping it simple is always the better course because it means there is less to go wrong. Nature can handle most things, but sometimes it need tweaking, like removing the nitrates.

Nitrates are probably your issue here. It would help to know the level in your source water. Fish are negatively affected by nitrates to varying degrees, but keeping nitrates as low as possible is always better for the fish. Other members have agricultural runoff nitrate issues too, so once we know the actual level one of them may have some suggestions. AbbeysDad has successfully solved his high source water nitrates problem, there may be others.

I've no knowledge of nor experience with the filter/filtration media you mention, so I cannot say if that will help or not matter. That may depend upon the nitrates and the solution to that.

I would not acquire more fish until this is resolved. Without fish in the tank it is easier to experiment with water chemistry and such.

By the way, rinsing the filter media under the tap is not bad, though with a new tank I would be more inclined to suggest a pail of tank water. But once things are established, under the tap is fine; I've been doing this for over 20 years. The fact is that nitrifying bacteria will not be killed off by tap water chlorine, in most cases; the chlorine has to be very high and over a prolonged period. Not that it matters here, as the bacteria have nothing to do with the unicellular algae. Nitrifying bacteria do not eat any algae.

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Fish Herder
Aug 8, 2015
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The benifical bacteria in your tank don't just live in the filter. They live everywhere in the tank. On the decorations, substrate, and glass. So just cleaning the filter with tap water should not harm an aquarium

To help you further we would need to know the size of the tank and what are your numbers from your tank water tests for Nitrate, Nitrite, Ammonia, and PH. We also need these numbers for for your tap water. It would also be helpful to know the GH, KH, and phosphate (if you know what it is) . If your don't know these number you can look on line for your water utility water quality report. That should list these items and many others. When you find the water quality report please post a link to it. so that we can go right to the site and see everything in it. There are also water tests from GH, KH, and phosphate but note all stores carry these tests (which was probably why the store clerk would not test for it. Below are to basic kit for GH/KH and phosphate.

I think you should look for the water quality report first That might provide all we need without you spending any money. Also how often do your change water and typically how much to you do.

It is generally said to be caused by nitrates, phosphates, ammonia/ammonium and in the presence of light.

Unfortunately this statement is very general. A lot of people have nitrates and phosphates in there water but most don't have green water and some don''t have any noticeable algae at all. So it might not be possible to from the water tests to say what is causing your problem. but it would give use enough to star eliminating some of the possibilities.


Fish Guru
Jan 26, 2008
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Perth, WA
Just wondering how the OP is doing with the tank or if they have given up.

If you're out there Emmyray, let us know what happened.

And for anyone else with the same issue, floating plants and daphnia.
Put some Water sprite in the tank and it will reduce light and use nutrients.
Live Daphnia will eat the algae and grow and multiply and the fish can eat the daphnia. Rotifers do the same thing.

If you have lots of nutrients in the water supply you could invest in a reverse osmosis unit and then add minerals to the R/O water to increase the hardness a bit.

And I would be drinking the R/O water rather than mains water if you have lots of nutrients in it.

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