Wednesday, December 17, 2008

Aquascaping Answers: Kelvin Ratings and Green Dust Algae

Alright, time to answer some aquascaping questions! I'll dive right in:

I was doing some research on the compact florescent bulb lighting system on my freshwater planted aquarium. (20 gal). I found out that I have this bulb. SmartPaq Daylight 10000ºK Actinic 460nm 40W I read online that this bulb is likely structured for a salt water reef tanks. Apparently actinic light is supposed to benefit corals. Other sources seem to suggest a K rating closer to 6700 for fresh water plants. Is this salt water bulb harming my plants? Would I get more results out of a 6700 bulb The best bulb available that I cant find is a 10000k/ 6700k 40 watt bulb. Will this make any difference? Thanks

First of all, you are not hurting your plants by using an actinic bulb. However, it's probably not doing them any good either. That's because plants use specific wavelengths of light for photosynthesis. These wavelengths are not produced by actinic bulbs, so yes, you would get more results out of a 6700K bulb. As to what Kelvin rating creates the best growth for plants, that's not the measurement you want to look at. Kelvin just approximates the color the light appears to the human eye, and not the wavelengths produced (though they do often correlate roughly). Therefore, it's much more helpful to get a spectral output graph for the bulb in question. Bulbs that match up with the wavelengths used for photosynthesis will probably be better for you plants. The differences may be too small to notice though, so if you can't find a spectral output graph,
just go ahead and buy a light in the 6700K-10000K range and it should put out the approximate wavelengths needed. That dual 6700K/10000K bulb sounds perfect. For more reading on the subject, check out my article Understanding Full Spectrum Aquarium Lighting.

My tank is relatively clean, free from algae, but I do get green dust algae on the glass. It stays off the plants, but I can't seem to keep it off the glass. Any recommendations? I don't believe I'm over feeding and I've cut my lights back a few hours. Is it just a matter of more regular water changes?

Ahh, the dreaded green dust algae. This one plagues even experienced aquascapers. What causes it is still not certain. Otherwise balanced tanks often experience it, so I wouldn't worry too much about reducing lights and feeding just yet. I'd try your hand at removing it. It's actually a zoospore, and this is important to keep in mind when trying to minimize the green haze it creates on aquarium glass. The usual tactic, scraping it off the glass with an algae scraper, won't work all that well, because this algae can swim. It will just swim right back onto the glass and re-attach itself. Weird, huh? So there are two ways to combat it. There's the not-so-patient method which involves scraping down the glass while doing a water change and sucking up as much of the algae as possible as you scrape it off. This has mixed results, but if done thoroughly and repetitively, should work. The patient method is to wait 2-3 weeks and not scrape it at all. This allows the algae to complete it's lifecycle, at which point it will turn into a thick jelly-like wrinkled substance. After 2-3 weeks, it will either drop off on its own, or you can safely scrape it off, sucking up the remains. Most people report this method to be most successful and the green dust algae does not return. I've managed to keep it in check with regular scraping and water changes and a handfull of Otocinclus catfish, but it's still there. I'm not that patient to let it grow out! Good luck!

Ask a question about aquascaping, keeping aquatic plants, plant-friendly fish, lighting, CO2, fertilizing, or any other aquarium plant related question in the comments to this post, and I'll answer them in next week's edition.


  1. That second question was mine. I actually had room in my tank, so I added some more Otos and a BN Pleco (still small). We shall see how they do, but I am also trying out the not scraping method. Once that's done I'll try to keep up with the regular water changes.

  2. For the 2nd question, I find I only get it when my tank is lacking in kh2P04.

  3. Can negative space be an effective focal point in an aquascape? What about driftwood - not the plants attached to the driftwood, but the driftwood itself (e.g., if you had an interestingly knarled piece)?

  4. for the dust algae, maybe you could try to let the water from your return pump drop onto a fine screen or a mesh (like the material used for small aquarium fish nets) before hitting the tank. this way the algae can grow on the mesh instead and not drift onto the glass of your tank. once the mesh is full, you could dispose the algae easily. (this works for marine/ reef aquarium algae)


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