# Does plexiglass block a lot of light?



## noobscaper (4 Dec 2020)

If I were to get a sheet of plexiglass as a lid for my 120x40x50 aquarium, how thick would it have to be? I'm assuming thicker than glass for the same size so it wouldn't bow. Unless it bows anyway?
Does plexiglass block a lot of PAR?


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## dino21 (4 Dec 2020)

Hi,

The table below shows most common glazing materials , and athough not listing a single sheet of Acrylic/ Plexiglass  its clear to see there is only a small drop when using most plastics.

However for your tank length you will need a support or two to stop it bowing, assuming you go for the typical 2,  3 or  4 mm thicknesses of Plexiglass.

Are you trying to keep things in or out ?  would a net cover work for you instead ?


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## zozo (4 Dec 2020)

noobscaper said:


> Does plexiglass block a lot of PAR?



Much less than glass, because it's much clearer if not 100% clear...  And clear glass still contains a percentage Iron/metal oxide which gives it that green hue, Optiwhite glass contains less Iron oxide and still has a slight turquoise hue in it.


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## noobscaper (4 Dec 2020)

zozo said:


> Much less than glass, because it's much clearer if not 100% clear...  And clear glass still contains a percentage Iron/metal oxide which gives it that green hue, Optiwhite glass contains less Iron oxide and still has a slight turquoise hue in it.


Would multiwall polycarbonate be better? That's my guess because it has air instead of plastic inside. 2x cheaper too...


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## dw1305 (4 Dec 2020)

Hi all, 


noobscaper said:


> Would multiwall polycarbonate be better?


I've used 10mm twin-wall polycarbonate roofing sheet, and it works pretty well. 

You need to seal the cut edges to stop water getting in and allowing algae to grow in the channels. I just taped the edges up with "10mm polycarbonate Sheet Aluminium Sealing Tape", but there must be a neater solution. 

cheers Darrel


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## zozo (4 Dec 2020)

dw1305 said:


> but there must be a neater solution.


It's as you say it's roofing material and there are standard cover strips/profiles available. But any hardware store might have plastic/pvc/aluminium U or L profile in stock... If it's 10mm sheet, then a 12mm U or L profile is at least 10mm internal and fits snugly over it, then glue it or screw it on. 

This picture of sheet closure is from a Polycarbonate sheet shop. Also a simple U profile, but it could have an extra internal ridge to clamp it to the sheet.


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## tiger15 (4 Dec 2020)

Even the cleanest glass or plexiglass top will cut off 20% light. Plexiglass is slightly more transparent than glass because it reflects less light, but you need greater thickness to prevent sagging which offsets greater material transparency.  The difference in material transparency is really insignificant in comparison to what dirt and algae built up on the cover will reduce light.  I have glass top which I clean every week in WC or else I will lose over 50% transparency within weeks.  I compensate light loss partially by tying some submersible LEDs inside the rims to bypass the cover blockage.

If your objective of the top is to prevent fish jumpIng out, open egg grate is most transparent with no decrease in transparency from dirt over time.  Glass or plexiglass tops have  the advantage over open top or egg grate though in that they can reduce evaporation and CO2 escape. .


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## noobscaper (4 Dec 2020)

dino21 said:


> Hi,
> 
> The table below shows most common glazing materials , and athough not listing a single sheet of Acrylic/ Plexiglass  its clear to see there is only a small drop when using most plastics.
> 
> ...


Sorry for the late answer - I couldn't see your answer before for some reason!
I'm trying to do both. Lots of stray balls flying around nearby


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## oreo57 (4 Dec 2020)

To sort of begin at the beginning:
Extruded plex warps easily.
Cell cast does not.
Thickness doesn't really matter except fir rigidity

Most say twin wall polycarbonate is stable.
Also easy to work with.

Most plastics have a sharp cuttoff in transmission below 400nm. Not normally an issue in fw lights.

All lose some light though most are in the 10-15% range. Even "dirty".

Another option us eggcrate plastic.
You need to define your needs.
Lower evaporation?
Structure strength?
Fish jumping proof (what size fish).
As mentioned there is also netting material.
Size and color matters.
Globally most materials have been quantified to their light characteristics.
Worst common cover for light transmission would be fine mesh black screen. Which us sort of obvious.







Thickness does matter a little.
Polycarbonate absorbs more visible light than plex.


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## noobscaper (4 Dec 2020)

oreo57 said:


> To sort of begin at the beginning:
> Extruded plex warps easily.
> Cell cast does not.
> Thickness doesn't really matter except fir rigidity
> ...


I'd like to lower evaporation, prevent jumpers (smallest fish are 1") and need it not to bow.


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## oreo57 (5 Dec 2020)

Glass is your overall best choice in my opinion..
I can't vouch for  no bowing w/ cell cast but can guarantee it w/ other acrylic types.
Acrylic Aquarium Cover - Page 2 - Aquarium Advice - Aquarium Forum Community
Dual wall Polycarbonate is fine  w/ more light loss than the above types.
DIY Aquarium Lids - Polycarbonate - Odin Aquatics
Solid sheets seem pretty stable but if too thin will droop.
I'd probably throw out acrylic just because..

so glass or polycarbonate


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## noobscaper (5 Dec 2020)

oreo57 said:


> Glass is your overall best choice in my opinion..
> I can't vouch for  no bowing w/ cell cast but can guarantee it w/ other acrylic types.
> Acrylic Aquarium Cover - Page 2 - Aquarium Advice - Aquarium Forum Community
> Dual wall Polycarbonate is fine  w/ more light loss than the above types.
> ...


Looks like I'll go with the polycarbonate then.
Glass was... Well...
Let's just say I had a bad experience with a glass cover not so long ago - and cleaning tiny shards off the floor isn't my idea of a good way to spend the weekend


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## dw1305 (5 Dec 2020)

Hi all, 
This is the lab. tank with the twin wall polycarbonate cover. Apologies for the quality of the photos.








cheers Darrel


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## oreo57 (6 Dec 2020)

noobscaper said:


> Looks like I'll go with the polycarbonate then.
> Glass was... Well...
> Let's just say I had a bad experience with a glass cover not so long ago - and cleaning tiny shards off the floor isn't my idea of a good way to spend the weekend


Well you can always get it tempered then you will only be picking up little squares of glass..


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## noobscaper (6 Dec 2020)

oreo57 said:


> Well you can always get it tempered then you will only be picking up little squares of glass..


Ye I had it tempered. Not fun.
The thing disintegrated in my hand in slow motion 👌
At least it didn't injure me too seriously. I think I still have a splinter inside me though...


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