# Will glass or plexiglass diminish a light source's effectiveness?



## Ligyron (Mar 6, 2016)

IOW, will a clear glass tank cover between the light source and the water surface make that light any less effective relative to our plant's needs? Thanks!


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## mistergreen (Mar 3, 2007)

Yes. You can get stronger lights to compensate.


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## tiger15 (Apr 9, 2017)

Yes, even the cleanest glass can block off 20 to 30% light intensity, and the percentage reduction goes up if it is dirty or algae coated. This is why competitive aquascapers use open top tanks. But if you keep fish that jump, you may not have the choice, One advantage of glass top tank though is better containment and redistribution of CO2 in the air space.


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## mistergreen (Mar 3, 2007)

Yes, also reduces evaporation.


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## Seattle_Aquarist (Mar 7, 2008)

Ligyron said:


> IOW, will a clear glass tank cover between the light source and the water surface make that light any less effective relative to our plant's needs? Thanks!


Hi @Ligyron ,

I believe that @tiger15 may have accidentally overstated the effect of using a clear glass top between the light source and the tank. Here are some actual numbers taken with a calibrated Apogee MQ-510 PAR meter on my 20 gallon tank. 
[email protected] (dirty glass top) 
[email protected] (clean glass top)
[email protected] (no glass top)

I've done this test over several different tanks over the last several years. Typically I find a -10% drop from a no-top tank to one with a clean top and if I don't keep my glass tops clean over several months I lose another 10% due to dust and water mineral build-up. I always use a glass top - fish jump and water evaporates. Hope this helps! -Roy


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## jake37 (Mar 18, 2021)

Good info. Nice and precise. Thanks.



Seattle_Aquarist said:


> Hi @Ligyron ,
> 
> I believe that @tiger15 may have accidentally overstated the effect of using a clear glass top between the light source and the tank. Here are some actual numbers taken with a calibrated Apogee MQ-510 PAR meter on my 20 gallon tank.
> [email protected] (dirty glass top)
> ...


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## tiger15 (Apr 9, 2017)

Seattle_Aquarist said:


> Hi @Ligyron ,
> 
> I believe that @tiger15 may have accidentally overstated the effect of using a clear glass top between the light source and the tank. Here are some actual numbers taken with a calibrated Apogee MQ-510 PAR meter on my 20 gallon tank.
> [email protected] (dirty glass top)
> ...


Thanks for the correction. I cited off my head from reading Karen Randall Sunken Garden long time ago and rechecked that she said 20% reduction for the cleanest glass. But your figures, 10% for clean glass and 20+% for dirty glass, should be the standard as you provided first hand data.

But I’m wondering whether your measurement is based on led or T5, having a reflector or not, and whether the light sits on top or at some distance above the cover. There may be some influence from the angle of light reflection and deflection on glass top.


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## Seattle_Aquarist (Mar 7, 2008)

tiger15 said:


> Thanks for the correction. I cited off my head from reading Karen Randall Sunken Garden long time ago and rechecked that she said 20% reduction for the cleanest glass. But your figures, 10% for clean glass and 20+% for dirty glass, should be the standard as you provided first hand data.
> 
> But I’m wondering whether your measurement is based on led or T5, having a reflector or not, and whether the light sits on top or at some distance above the cover. There may be some influence from the angle of light reflection and deflection on glass top.



Hi @tiger15 

Actually, over the years I done the test with both Power Compact (basically T5HO) florescent with MIRO 4 stainless reflectors and more recently with LED fixtures. My fixtures have always sat on the top of the tank just a fraction of an inch above the glass tops. This probably gives the most accurate estimate of the drop in PAR; I agree if the fixtures are raised the amount of PAR drop may be different - I've never tried it. Tops are Aqueon Versa-tops. -Roy


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## tiger15 (Apr 9, 2017)

Seattle_Aquarist said:


> Hi @tiger15
> 
> Actually, over the years I done the test with both Power Compact (basically T5HO) florescent with MIRO 4 stainless reflectors and more recently with LED fixtures. My fixtures have always sat on the top of the tank just a fraction of an inch above the glass tops. This probably gives the most accurate estimate of the drop in PAR; I agree if the fixtures are raised the amount of PAR drop may be different - I've never tried it. Tops are Aqueon Versa-tops. -Roy


Having a reflector and sitting on top of the glass will recapture more of the reflected light. Having the light hanged above the top and not having the reflector will reflect away more light leading to higher net loss.


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## Ligyron (Mar 6, 2016)

Ligyron said:


> IOW, will a clear glass tank cover between the light source and the water surface make that light any less effective relative to our plant's needs? Thanks!


Thanks to all for such good information and an interesting discussion. The answer in my case is to just use a stronger light to compensate as MisterGreen proposed whether it's over a tank with lively fish or an emersed culture container of some sort. I hadn't considered the conservation of CO2 aspect but a tight fitting top must surely help with that to some extent although I should imagine measuring how much would be a real challenge. Not even going there, lol.


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