Dispelling the myths about glass lids on reef aquariums.

[Dburr1014]

I was shocked to see that clean glass did not meaningfully reduce my par (ai prime light, apogee 510 par meter). I was equally shocked to see how badly a plastic substitute decreased par. I forget if it was lexan or something else. Some clear plastic (plexi glass?? I hesitate because these are all brand names). The plastic I used decreased par by like 50%. But I believe SOME plastics do not. The fancy screen lid manufacturers sell plastic inserts, so they must have found something that works.

50%???
I find that really hard to believe.
Do you have a link?
I have lexan from a old metal halide fixture I butchered up I use on my led photon v2. I installed it on my light itself and it does get a little bit of spray that I clean every 2 or 3 weeks. It's not terribly bad but still needs to be touched up. I need this cuz I run with no lenses installed.
I did used to keep them on the tank but they would get water drips inside from condensation. I think that is far worse than what I do now as far as par is concerned.

 

[Chrisv.]

50%???
I find that really hard to believe.
Do you have a link?
I have lexan from a old metal halide fixture I butchered up I use on my led photon v2. I installed it on my light itself and it does get a little bit of spray that I clean every 2 or 3 weeks. It's not terribly bad but still needs to be touched up.
I did used to keep them on the tank but they would get water drips inside from condensation. I think that is far worse than what I do now as far as par is concerned.

I measured it with a par meter myself, so there is no link. Might have been only 40% loss. also I’m quite sure that this varies from brand to brand of clear plastic, with some formulations specifically designed to reject heat and uv. I don’t know that the brand I used was Lexan, only that it was whatever Home Depot was selling s precut sheets that met my needs.

the difference was quite striking. I still had the glass lid on hand and even the relatively dirty glass lid transmitted more than the plastic.

Looks like CTARS has a par meter— why not measure and let us know if the brand you’re using has a big impact on par?

 

[sfin52]

The biggest plus for me is reduced evaporation.
The biggest minus is trapping heat.

I use glass lids to trap heat and lower evaporation.
My tank sits in the basement and in the sumer it's OK. Basement stays 68-70. During the winter temp drops to 65.
Since I have a basement I run a dehumidifier all year. Even if you run a ac the humidity in the basement can be high. Also the dehumidifier helps with insects control and spiders.

 

[Reefinmike]

IMO, while tank tops do have substantial gas exchange, so do skimmers and unless you treat the skimmer air, the glass top effect may not be as definitive as you suggest.

Im also not sure you are not loosing some oxygenation. Have you ever measured O2 at night?

Agreed, this statement only applies to a tank with no skimmer, a skimmer recirculating it’s own air or using a co2 scrubber. No way glass lids could make a significant effect if the skimmer is drawing room air

 

[Reefinmike]

Everyone is entitled to their own oppinion. I dissagree with most of yours however. Glass lids are fine for FOWLER tanks but I much prefer an open top for a reef tank.

1) I live in a pretty dry hot climate so a little extra humidity in the house is more than welcome. I also use evaporation as a cooling mechanism in the warmer months.
2) Increasing your light output to compensate for dirty glass is a waste of electricity.
3) I have wrasses in my open top tank and never had one jump out. I believe a eouo brace helps mitigate much or the risk.
4) My pH tsts between 8.2 -8.4 24/7 so CO2 is not really a concern.
5) My lights are shileded with glass, so splashing is not really a concern for me.

If glass lids work for you AWESOME... you do you. As for me, I won't be using one.e

I plan on addressing your second point in my next experiment using an anbient room temp probe, three tank temp probes and monitoring the wattage draw of heaters over the course of a few days with vs without lids. In my example, the lights only need an extra 42w to overcome the par drop from glass. I have a hunch that a topless tank will require a lot more extra heater juice sitting in a 69*F room.

 

[Dburr1014]

Looks like CTARS has a par meter— why not measure and let us know if the brand you’re using has a big impact on par?

It just so happens in may I did check my tank with a par meter. I checked with dirty lexan, clean lexan and nothing. Problem is I didn't save my numbers. At the time I was just cranking up the light and get par as high as I could with it still looking nice to my eye.
I don't remember a wildly difference between the 3.
I wish I could get her hands on the meter. CTARS has been having trouble getting the club together and the guy that actually had the meter isn't answering the phone.

 

[EeyoreIsMySpiritAnimal]

I use glass lids to trap heat and lower evaporation.
My tank sits in the basement and in the sumer it's OK. Basement stays 68-70. During the winter temp drops to 65.
Since I have a basement I run a dehumidifier all year. Even if you run a ac the humidity in the basement can be high. Also the dehumidifier helps with insects control and spiders.

In Houston and there's no way I could afford to keep my house 68-70 in the summer! I do use that glass in winter to reduce heater use.

 

[sfin52]

In Houston and there's no way I could afford to keep my house 68-70 in the summer! I do use that glass in winter to reduce heater use.

Oh that's my basement my main floor is 76.

 

[EugeneVan]

I like to smell my tank.

True, once you owned a reef tank for more than a year, you can tell what is going on in the tank by the smell

 

[Chrisv.]

It just so happens in may I did check my tank with a par meter. I checked with dirty lexan, clean lexan and nothing. Problem is I didn't save my numbers. At the time I was just cranking up the light and get par as high as I could with it still looking nice to my eye.
I don't remember a wildly difference between the 3.
I wish I could get her hands on the meter. CTARS has been having trouble getting the club together and the guy that actually had the meter isn't answering the phone.

ah the plot thickens! As I think more carefully about this, I am not at all surprised that you have a different experience. If im not mistaken, you said your plastic came from an old aquarium light (probably selected originally to transmit light) and mine came from a home improvement store-- probably designed to filter UV.

I know for sure that the fancy mesh lid companies sell inserts and I believe they don't screw up par. So I think this is a situation where the devil is in the details. I think that not all lexan/plexiglass/acrylic/polycarbonate is created equally, and I expect that is the basis of our different observations.

I am actually getting a new light this weekend, so I will get our local club par meter to dial it in. When I do, I will make a quick video showing what I'm talking about with my plastic lid and send it your way!

 

[Lavey29]

True, once you owned a reef tank for more than a year, you can tell what is going on in the tank by the smell

Totally, it's the first thing I do in the morning.

 

[FishOkay]

I put it to you that your dirty glass wasn't that dirty blocking only an additional 2% of light lol. But in all seriousness I'm sure with salt creep and the water droplets/condensation it's more than that. I had glass lids but got rid of them, too much cleaning and to much of a greenhouse effect for my liking. Yes they keep fish from jumping but not many other reasons for it over something like a jump guard imo. You made some good points though. I also wouldn't say they are myths needed to be dispelled

 

[rtparty]

10% PAR loss from glass is already known. I ran this experiment about 14 years ago with ATI T5 fixtures. Losing 25-30% of your PAR was not unexpected once we coated the glass in some salt creep and water droplets. I would provide the link but that is BURIED in the T5 Q&A Thread over on RC. No idea how far you would have to go back to find it.

The acrylic shield that ATI uses on their fixtures was the best ATI could find and it limits 6-7% of the PAR which is better than any glass they could find and test. I spoke directly with Oliver about this years and years ago.

The truth is, there are a million ways to run a tank and every system is so different. Anyone claiming Mike's points aren't valid because they don't fit THEIR setup is flat out wrong and not understanding the the thread and points made

 

[Greybeard]

They prevent fish from jumping
-Some sort of jumping protection is a positive, a mesh top provides the same benefit.

They limit evaporation reducing your need for topoff water- expanding the duration your ato reservoir lasts.
-I'm a Kalkwasser user. More evaporation is a good thing, helps maintain better calc/alk/pH levels. My ATO and Kalk reactor reservoirs are hard plumbed, haven't manually filled a reservoir in years.

They limit humidity being added to your household air
-A wash... half the year, this is a positive, half the year, a negative. Again, evaporation is a good thing.

They limit evaporative heat loss allowing you to use much smaller heaters(risk mitigation) and save a significant amount of electricity($$$)
-A properly designed heating system, say an independent heater controller plugged into an Apex, is much better for risk mitigation than an undersized heater. A properly sized heater, running over less time, is more efficient than running an under sized one longer... and again, evaporation is good

They protect your expensive lights from rogue fish splashing
-Never had a splash problem with my lights, but I do run my lights a bit higher than many, at 14" or so over water level

Most importantly- they limit the effect that high CO2 household air has on your pH. Reducing gas exchange reduces carbonic acid addition to your reef.
-Gas exchange is a huge positive, in my opinion. Co2 is _released_ from the aquarium via gas exchange with the atmosphere, being a normal product of photosynthesis. If your pH is low, Kalkwasser, which requires high rates of evaporation for efficient use, boosts pH very effectively.

I'll stick with my mesh top, thanks.

 

[Randy Holmes-Farley]

Yes absolutely the skimmer has the largest contribution to gas exchange.

Even if the skimmer is drawing outside air, it’s still entraining a consistent 410ppm of co2 into the water. Believe it or not, there is a point where outright removing said skimmer has a positive effect on pH. I’ve been toggling dosing additives automatically between Lye, soda ash and bicarbonate for about 2.5 years now. Removing my skimmer 18 months ago was one of the best moves I made for my reef. The pH rise was admittedly minimal- 0.07-0.10 but coral health and growth increased significantly.

You have no skimmer AND a closed top, and are not concerned about low O2 at night?

Ever measure it?

 

[diabolical_clownfish]

Seems like it boils down to the following:

low tank PH + low ambient CO2: gas exchange is good, increases pH
low tank PH + high ambient CO2: don't care, gas exchange is not helpful
high tank PH + low ambient CO2: don't care, gas exchange is not helpful
high tank PH + high ambient CO2: gas exchange is bad, lowers pH

So if I'm happy with my tank pH and don't dose Kalk, etc, then getting a glass lid is a good idea (limit evaporation, etc)

 

[Peter Houde]

Very interesting. I never would’ve thought polycarbonate could have that much of an effect.

Some window glasses have light filtering films to limit UV and heat energy entering a building. I wouldn’t be surprised if some plastics intended for windows have similar properties.

A lot of people think polycarbonate and acrylic and plexiglass are the same things. They aren't and they have very different properties. I use polycarbonate over half my aquarium to cut down on evaporation and more importantly to shield my lights from the mist generated by the sump return. I clean it weekly with water changes. I leave the other half open to reduce the load on my chiller and for the convenience of feeding. I have to admit that I've never measured PAR, but I can't see any difference between one side of my aquarium and the other either visually or in terms of what grows.

 

[Mark Gray]

People in the hobby often repeat what they hear others say-
- “Glass lids Are bad. They get dirty and block a lot of light”
- “Clean glass drops PAR 10-20%. On top of that, the dirt, salt creep and humidity drops cut par an another 10-15% so you have to increase your lights every week to account for this then back to baseline when tops are cleaned”



I’ve been using glass lids for 17 years and strongly feel their benefits far outweigh the par loss most modern lighting is able to overcome.

• They prevent fish from jumping
• They limit evaporation reducing your need for topoff water- expanding the duration your ato reservoir lasts.
• They limit humidity being added to your household air
• They limit evaporative heat loss allowing you to use much smaller heaters(risk mitigation) and save a significant amount of electricity($$$)
• They protect your expensive lights from rogue fish splashing
• Most importantly- they limit the effect that high CO2 household air has on your pH. Reducing gas exchange reduces carbonic acid addition to your reef.

I did a little 1 hour real world experiment using 3 new Neptune sky lights mounted over a 72”x18”x27” 150 gallon reef with admittedly yellow foggy water(I don’t use any filtration whatsoever). The bottom of the light is 3.0” above the glass, 5.0” above the water surface and 16.5” above the par sensor(11.5” under water). The par is measured with the neptune PMK(apogee sensor). The glass is a mix of what fits. Measurements were taken under the center 3.9mm thick piece of very scratched up tempered green glass. My current lighting is set to Neptune’s custom “sky” color recipe at 66% intensity. Readings were viewed without lag using the display module. All pumps were left running as usual and the average was taken after 5 minutes. My wav pumps are on 5 second alternating pulse so flow anomalies weren’t a factor.



SKY 66% - PAR - % of Max

Dirty glass - 227 - 89.0%

Clean glass - 232 - 91.0%

No glass - 255 - 100%



Sky 100%. - PAR - % of MAX

Clean glass - 330 - 89.2%

No glass - 370 - 100%



All ch 100% - PAR - % of MAX

Clean glass - 368 - 89.8%

No glass - 410 - 100%



In conclusion, clean glass cuts PAR by 10%(on average) vs no lid at all. Dirt buildup reduces PAR an additional 2%. Using my 66% sky setting as an example- if I wanted to emulate the par with no lid I just have to bump up intensity 7% to 73%. Glass lids work.

I believe this squashes the misconceptions surrounding PAR reduction however it does not address the potential shift in color spectrum. I’ll leave that up to someone who really wants to nit pick lass works for me. In another, much more thorough experiment I will address the pH “boosting”, topoff reducing and electric(heat) saving benefits of glass lids.





Image description

1- shows the amount of dirt allowed to accumulate on lids over 6 weeks

2- shows the general hazyness to the water. And wild coral growth provided glass lids don’t hold you back

3- par sensor location

4- shows the basicness of the setup. Random glass lids, hastily made light rack resting on two stacks of buckets- real world testing.

5- This is the center glass that readings were taken under. This is it CLEAN. Very hazy, scratched, hard water deposits

Well I have used glass lids on a few tanks. I have never used them since I started with coral, maybe 6 to 8 years ago, I have never done a par test with them. Personally I hate glass lids, I have been in the hobby a little while, since the 80s. I can remember how much brighter the tank was when I was done. So to me they are a huge amount of extra work. My house is not air tight so PH is not an issue for me. If my ATO water runs out I have a much bigger problem than evaporation, I have a 100 gallon reservoir. Good Luck

 

[Chrisv.]

You have no skimmer AND a closed top, and are not concerned about low O2 at night?

Ever measure it?

Haha for a while I was running a closed top with no skimmer and no air stone. I am ashamed to say that there was a huge behavioral difference in my fish when I added air. Nobody died, but they were for sure hypoxic. It was a pair of clownfish and given how stupid they are already, it's impossible to know if there was brain damage lol.

 

[Chrisv.]

A lot of people think polycarbonate and acrylic and plexiglass are the same things. They aren't and they have very different properties. I use polycarbonate over half my aquarium to cut down on evaporation and more importantly to shield my lights from the mist generated by the sump return. I clean it weekly with water changes. I leave the other half open to reduce the load on my chiller and for the convenience of feeding. I have to admit that I've never measured PAR, but I can't see any difference between one side of my aquarium and the other either visually or in terms of what grows.

I'm sorry-- I know these are different, but I don't understand what the difference is. I get that they have different chemical makeup, but I don't know when to use one versus the other. Would you please explain?