So my 45 gallon tank water has a greenish tinge to it. I assume its algae, as I put it below a window. Even with the blinds closed, Im sure it gets some sunlight causing the algae to grow. I got a green killing machine to try to fix the issue, however its been 9 days and I see no visible change. It says it works for tanks up to 50g, so it should work. Do I just need to let it run longer? Or perhaps I need to get a larger capacity model like this?

  • 1
    Can you post how old the tank is, stocking level, and what your maintenance/water change procedures and schedule?
    – Jestep
    Commented Sep 22, 2017 at 18:36
  • @Jestep the tank is new, about a month and a half old. Ive only done a few water changes. Twice, doing about half, which doesnt help much at all.
    – Keltari
    Commented Sep 22, 2017 at 22:19
  • I once solved a green algue problem by putting daphnia in a small container inside the aquarium they'll not populate that fast, but reliably solve the problem and afterwards make a great food source. Commented Sep 23, 2017 at 10:24
  • My experience with algaecides is that they kill plants better than they kill algae , Specifically hornwort . Commented Sep 24, 2017 at 1:24

3 Answers 3


As a general diagnosis of the situation, it sounds like what you have is an algae bloom. Algae grows when there is sufficient nutrients and light in the tank which is not being used by anything else.

UV Steriliser

You've already got a UV steriliser which can help to combat the problem but is not the only way. The key to a UV steriliser is the amount of time the water is exposed to the UV lamp. If the water is flowing too fast through it, the lamp will have very little effect. Before upgrading, and spending possibly needless money, there are some other things that you can check first.

Test Kit

Get yourself a water quality test kit if you haven't already and check that all the levels are adequate. Ensure that there is no ammonia, no nitrite and somewhere in the region of less than 30ppm nitrate (depending on your fish of course). An imbalance in your nutrient levels could trigger an algae bloom.

Light Levels

If you've got extreme light you open yourself up to the possibility of an algae bloom. The light alone is probably not the cause and you'll likely discover it during the 'test kit' stage. Generally, you should avoid putting a fish tank near a window because it means you cannot control the light that enters your tank which can be a problem. If this isn't an option, read on for more ways to reduce the chance of it becoming a problem.

Carbon Dioxide Levels

Just like any photosynthetic organism, they need carbon dioxide to photosynthesis. If you've got a high level of carbon dioxide in your tank, you've got a great environment for maximum algae growth.

Okay, okay. I get the point but how do I prevent algae blooms?

Glad you asked. The best way is to make sure that you provide a bad environment for algae growth.

Decrease light level

If possible, move your tank away from the window or decrease the amount of time that your fish tank light is on for during the day.

Decrease carbon dioxide concentration

Add some kind of air stone and air pump to increase the oxygen levels in the water and help gas off the carbon dioxide stored in the water. You can also improve your aeration by ensuring there is a good level of surface movement in your tank.

Regular Water Changes

Make sure you're doing your regular water changes. Water changes help to decrease nitrates. Nitrates are a great plant food and is what helps a planted fish tank thrive. Make sure you remove an uneaten food and you're not over-feeding your fish because these all contribute to the nitrate levels increasing.

Get some real plants!

If the nutrients in your water get used up by something else, the algae will have a tough time getting a hold of your tank. There's some really easy to grow plants:

These will help to suck the nitrates out of the water and use the carbon dioxide in the tank. If you don't fancy having plants in your tank, you can always let the roots of a plant trail into your tank which will work the same. Here's a great instruction video about growing Pothos (devil's ivy) in the top of your tank.

Additional Reading:

PS. To fix your current situation, you'll probably need to increase your water change percentage for a little while until you can remove the algae that already exists in your tank.

  • I already know its algae. your lengthy answer doesnt answer my quiestion. Its just a rehash of easily found googleable online info.
    – Keltari
    Commented Sep 23, 2017 at 10:26
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    @Keltari I've actually collated this over my years in the hobby and from my own personal experiences with dealing with algae. I cited sources to confirm what I'm saying is correct. Your question says 'I assume it's algae' so I wanted confirm you are correct in that assumption. My answer also explains about UV sterilisation which is what you asked about and explains how exposure time is key and that getting a stronger lamp may not help. If that's not question, could you please clarify what your question is?
    – Henders
    Commented Sep 23, 2017 at 10:29

I've been in this case before around 3 years ago. The only thing I did before was moving it to a different location where my tank won't exposed by any light that came from the sun. I didn't have to use any items since it's kinda a waste of money if there any solutions that you can just do without spending a money.

You can check my answer on the link https://pets.stackexchange.com/a/18591/10116

Just try to move it to another location and observe.


I spent a lot of money on getting one of these for a 29 gallon volume, for a friend whose tank I tend to when I can. He is a fire-fighter on a military base, working 24 hour shifts, sometimes 11 days in a row.

When I first started helping him out he had this same problem. Thinking I wouldn't spend much time and effort at his place I bought him a UV sterilizer rated for up to 30 gallons. I didn't have much luck with it treating the algae bloom, but it worked great after getting rid of the bloom in prevention/preventative care.

He overfeeds his pleco sinking algae disks with the idea that if he feeds a bunch and then doesn't feed him for days while he's working that it's his best option. Although I don't agree with this concept, he still does it. But, after I turned his lights off, and covered his tank with a blanket for 3 days, the algae disappeared and now the UV sterilization has prevented an additional bloom.

He also has no option to move his tank (room size and space and outlets). I would say your size is adequate, you won't want to go much larger aka more powerful, but may want to try a combination of the answer above, and reduce feedings of foods with algae in it and keep the UV sterilization as a prevention method for future blooms.

Hope this answers your question. The previous answer is very informative for any reader with algae issues in their tank.

  • Also I read the description of this specific brand. It says it has a LED light that signals when you need to replace the UV... It shouldn't indicate being out, since it's new, but have you checked the indicating light? Also, I'm just curious, how's the power head working?
    – Christy B.
    Commented Sep 25, 2017 at 17:43

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