Does Plexi-Glass cut down light emitted into tank ?

[Kflash1]

Plexi-glass is actually 7 times clearer than glass. That is why when you look into a plexi tank that it almost looks 3D like the fish are going to come right out to you. As far as cutting out the light spectrum I don't believe it will but someone might have more knowledge on that then I do. As long as you keep the salt cleaned off of it I would believe that it would be good protection for your lights.

Kirk

 

[AnotherGoldenTeapot]

Yes it will reduce the amount of light hitting the water by quite a bit. Whether you notice is another matter.

I can't locate the numbers for acrylic, but the numbers for clear polycarbonate are:

3mm - 86% of visible light (almost no UV)
4.5mm - 85%
6mm - 83%
9.5mm - 79%
12.7mm - 75%

 

[odenwell]

Polycarbonate is not a good comparison to acrylic. If you keep the acrylic clean then I don't think it will block much light at all. I don't like tank covers because I think it tends to cut down on the amount of oxygen in the water. This may not be an issue if you have good circulation and a sump. A small hang on the back skimmer is marginal when it comes to providing oxygen.

 

[I14Green]

Should I remove the 1/4" plexi-glass top on my tank and go with nothing. My reservations are : the risk of shattering my bulbs. Also I will have to clean them off once a week due to salt build up.

Questions:
Does the plexi glass cut the good ( right spectrum)light out significantly or is it inappreciable. I don't notice a difference in brightness when I remove the plexi. I have 1-32W double P.C. unit and 1-24" Flourescent Actinic Bulb. I originally put the plexi on because my bulbs were getting salty and I moved them about 4" from the top of the water to gain more light penetration. I have a 15 Gal hex tank.

 

[I14Green]

OK more input needed please ?!? But on another topic how close can my 32 W PC bulbs be to the water, how much does splashing effect the bulbs !

 

[AnotherGoldenTeapot]

Light transmission for 6mm acrylic is 92%.

 

[tim000]

I would like to know also. I have my PC's about 4" from the top of the water, and they get covered with salt. i was thinking about adding some plexi-glass but didnt know if that was a good idea.

 

[lawndoctor]

This topic has generated some debate on this board in the past, so you might want to do a few searches and check out the differing opinions. Also, one of the messages in these discussions listed a link which provided specific research on light transmission/absorbance by a variety of materials which could be used as covers. Hope this is helpful.

 

[I14Green]

LawnDocter - do you know any of those websites URL's ?

Also i would have to say that my star polyps in my tank not doing as well as they were with no plexi on the tank. I think this could be from a number of things : 1) Excess heat traped in tank causes temp to rise. This is happening - tank is up to 80 Degrees Almost - when I'm usually around 77 degrees. 2) Lack of Oxygen exchange. Carbon dioxide also trapped under tank lid. 3) Salt and other mineral deposits are clouding Plexi at an alarming rate reducing light passage. As well as the fact that Scrubing plexi causes it to become less clear over time. Both are happening to me also. WHAT DO I DO !???! PC light need to be close to tank top ( 4" or so right - at 6" it was not close enough) Any ideas and/or input into this debate are appreciated ?

 

[AnotherGoldenTeapot]

Simply ask for the technical specifications for the brand of acrylic you plan to use. Light transmission will be one of the characteristics reported. The answer is definitive, if you believe the manufacturer.

Alternatively you can try a seach on "acrylic light transmission" etc.

 

[lawndoctor]

I did a number of searches, but for some reason none of my searches went back past June 2001. The thread I remember was before that. Sorry, but I don't have the web site. For what it's worth, I cover my tank with 3/16" plexiglass based on research I did a few years ago which showed that plexiglass, unlike glass, blocks out most of the worst UV and lets most of the rest through. I understand that is why museums use plexiglass to encase exhibits. Plus it's dirt cheap and is easy to cut. My VHOs are only about one inch above the plexiglass, so going without a cover is out of the question. I run a small bubbler in my sump to provide additional oxygenation and carbon dioxide gas exchange. I have never had a problem with oxygen levels, but I did have a carbon dioxide problem before I put in the bubbler. No problems since. I rinse the covers weekly, but never scrub them. Also, I have the corners cut out of the covers above the overflow so I can get a grip on them to remove them and so some air gets through. Turning the lights off does not seem to lower the temperature because my chiller operates at about the same intervals day and night. However, I did notice that removing the hood entirely does keep the temperature down. So the hood and covers do seem to act as insulators. In the future, I would consider going with UV-shielded pendants hanging high enough to keep the tank top completely open, which I think would look really cool, as long as there was no problem with jumpers. For now, VHOs with covers work well for me. Good luck.

 

[suckair]

UV-b blockage is a good thing! Covered systems are all I run. Low evap, no dust or dirt is introduced to the system, and the lighting stays clean! Some of the drawbacks are reduced light gets into the tank and chillers may be needed, as evap will no longer control heat build up. I feel these are acceptable cons and I love covered systems.


I would suggest that you use PPG's Starfire glass and have it tempered. Acrylic can warp and discolor under MH lighting. Starfire glass has very low light absorption levels and would be the best choice IMO. I am in the process of converting my systems to these tops.

Randall

 

[I14Green]

Thanks for all the help !

 

[rishma]

If you want to know the light transmission through PMMA (common cast acrylic sheet like PLEXI or LUCITE, you should consult the current ASTM (American Society for Testing and Materials) handbook. There is a standard light transmission test applied to clear materials. This test is performed on an (1/8)" thick sample and the values are published.

If my memory serves me correctly, Polymethyl-meth-acrylate has a transmission of around 92%. If your sheet is thicker than (1/8)" the relationship can be assumed to be linear for small increases in thickness.

As for any spectral shift, it will not be significant in the visible light range.