# Will glass top filter out useful rays?



## smittythefish (Jan 27, 2011)

Hello, 
I have a question I have been unable to find any info on.
I think glass filters out uv rays so i would assume we want no glass between our lights and the water.
But ive also heard it recommended that you have a glass top to protect your fixture from moisture.
both seem to make sense.
Whats the story?


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## JustLikeAPill (Oct 9, 2006)

Don't worry about it. It's up to you if you use glass or not and is really based on preference unless you use hqi lights which require glass. 

Keep the glass clean and there will be no problems.


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## HeyPK (Jan 23, 2004)

Ultraviolet rays are not useful to plants. They are outside the range of light used for photosynthesis.


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## Seattle_Aquarist (Mar 7, 2008)

Hi smittythefish,

First of all, welcome to APC! It is interesting you should ask since I just got done taking some PAR meter reading of my tanks both with and without the AGA/Aqueon Versa-Top in place. PAR is the abbreviation for Photosynthetically Active Radiation and and a PAR meter is used to measure to level of "useful" light.

What I found was the Versa-Top dropped the PAR value readings by about 10% (110 vs. 101). If the Versa-Top is dusty with dried water spots the drop is about 15% (110 vs. 96). If found the drop in PAR level was much greater from using older bulbs was much greater than the drop from using a glass top. Keep in mind that the "drop" in the PAR level is not really important as long as sufficient light for photosynthesis is reaching the plants all the way to the substrate level.

That said, I use glass tops on my aquariums and based upon the testing I have done I will continue. The benefits of personal safety, less fish loss from suicide jumpers, lower evaporation loss, and reduced fixture issues from corrosion outweigh the reduction in PAR level. Sufficient light all the way to the substrate level is typically not a problem I experience. I hope this helps.

30 Gallon; AHS 2X35 Watt Kit; PAR = 125 @ Subtrate


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## anubias6439 (Sep 7, 2010)

yes, clean glass is the key. I use vinegar and a razor blade if needed to keep my glass canopy clean about once a month.


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## spypet (Jul 27, 2007)

I had done tanks with and without glass tops.
the issue I have is not keeping glass clean,
it's keeping them condensation free.

often drops form on the underside of the glass,
only to rain back down on your tank water.
these disburse your lighting far more than 10%.

personally, the issue of covering is not to protect
your fixture, it's to protect your tank's inhabitants
from curious child hands, pets, and dopey visitors.


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## smittythefish (Jan 27, 2011)

Thanks for the prompt informative responses.
It seems that the difference in useful rays to the plants w/ or w/o a glass top is probably negligible
providing one is diligent about keeping the glass clean.
Why is it though that the _reef_ tanks i have seen always seem to be open top that is w/o glass cover?
I plan to install a retro fit kit in a home made hood for my 37 gal planted. Would you say people typically use a glass top in addition to the hood to prevent evaporation etc.?
Thanks again.


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## vancat (Nov 5, 2004)

Keeping it clean is key, because the glass will etch over time and NOT be cleanable.

I always had a glass top...but now love my open top! Evap is a non-issue for me....just add a gallon halfway thru the week. The open top is VERY CONVENIENT. And glass tops are expensive, too!


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## Seattle_Aquarist (Mar 7, 2008)

spypet said:


> often drops form on the underside of the glass,
> only to rain back down on your tank water.
> these disburse your lighting far more than 10%.


Just to clarify, the 15% drop in PAR value from an "open top" was with a glass top, that was a little dusty, had plenty of dried water spots, was somewhat "etched" from mineral deposits, and had condensation drops on the water side of the top.


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## spypet (Jul 27, 2007)

thanks for the clarification - Seattle.
my guess is the salts and calcium in reef tanks
accumulate far too easy on glass tops for them
not to quickly obscure your lighting, so reef tank
keepers forgo tops in favor of cavernous hoods.
smitty - do a search on any fish forum, and you'll
find this issue repeatedly under debate.


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## neonmkr (Jan 31, 2011)

One option instead of glass is to use polycarbonate (Lexan). I read, but don't remember where that you have greater light transmission with polycarbonate than with glass. Plus it's easy to cut aroung overflow tube, HOBs, etc. I've been using this for a few years and while I do not have a PAR meter to check it out, it does seem that my tank is brighter. As far as condensation, thats the only draw back but since my light fixture sits on top of the polycarbonate and not above it, the area under it is pretty free from water drops.


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## BryceM (Nov 6, 2005)

I agree that glass tops do cut down on available light (a bit), but, the light bulbs themselves are made out of glass...... somehow the useful light gets out.


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## ddavila06 (Jan 31, 2009)

i got rid of my glass tops because of the amount of calcium buildup...hated having to take them out, scrub scrub scrub and to only have to do it again one week later. i do get a large amount of evaporation though.


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