Acrylic lid vs mesh. What is the evidence behind oxygen depletion/CO2 accumulation?

[KonradTO]

Hi all,
I will give some more details to the title.
I recently started my adventure for the first time with saltwater aquaristic, so I have no N years of experience for supporting my claims.
I come from freshwater, and before that terraristic (bearded dragons, newts, frogs etc) and a common aspect of all my experiences is that I usually dive (A LOT) into forums like this one. I am a benthic dweller (LOL) so I usually read a lot and comment much less because I feel not confortable at giving advice when I am the least experienced.
Despite this, few times happened that majority of users kept suggesting like a mantra something which was obviously wrong, just because someone before them stated the same.

Coming back to us I recently red a lot of threads in reef aquaristic forums about using clear lids (glass or acrylic) where users most commonly adviced against using them and suggesting to use open tanks with mesh covers. The main reasons behind people claims were:
- Some wavelenghts being filtered by lids
- PAR being hugely filtered by lids
- Lack of oxygen in water because of lids
- Accumulation of CO2 (and pH swings) because of lids
- Difficulty in maintaining low water temperature with lids

is there any real evidence behind the first 4 statements as far as you know?
The only convincing one (to me, see above) seems the temperature being higher than normal, which can be easily controlled with fans or chillers.
- Spectra filtration: aren't light fixtures protected from splashes with acrylic/glass anyway? and as far as I know acrylic could filter only very small wavelenghts, much smaller than what we use in reef tanks.
- PAR filtration: Makes sense, especially if the surface is super dirty. But as far as I understood most of people run their fixtures with reduced intensity, so it would be necessary only to dim up the fixtures right?
- Lack of oxygen: Most of people are running skimmers, sumps and so on. Lids are never air-tight. It's like saying that if you stay in a room with only 1 window open you can finish all the oxygen. I can only understand this in case of super-crowded tanks where oxygen gets depleted anyway. As soon as there is surface movement of water it should be fine right?
- Accumulation of CO2: As far as I understood that is entirely dependent on the CO2 levels inside the room and CO2 will balance with the CO2 outside the tank anyway.

On the other side having no lid increases HUGELY evaporation--> necessity to buy an ATO (freaking hobby is draining my savings)--> Necessity to fill the ATO reservoir-->if ATO fails you can flood your flat AND kill your fish.
Also increased humidity in a flat can lead to problems with molds, which can be very dangerous for people's health.

What's your thoughts on this?

 

[blaxsun]

I have diffusers on my Radions, which effectively acts as a splash guard. While I'm sure the lights could tolerate a degree of saltwater, I'd prefer they didn't. Yes, a lid would minimize this - but I'm not sure I'm ok with the PAR, wavelength and other tradeoffs.

The biggest reason I don't like solid lids is the salt creep, restricted access and additional maintenance. Scraping my tank every week is bad enough already without adding to my chore list.

Skimmers and powerheads help to be sure, but if you have a closed sump and stand - airflow is still somewhat restricted. Nothing really beats a large open surface of water.

 

[mrlavalamp]

I have always used a glass lid on my 40B quarantine tank.

I did not have any issues with lighting spectrum or power and keeping corals in the QT for the few weeks before they went into display, it was not an issue at all. I think this would really only be an issue in deep tanks, extremely demanding corals, or if you are using a very low power light. I did have to clean them weekly, but that was just a quick rinse in tap water, walking to and from the sink took longer than the actual rinsing so I dont see it as a huge maintenance issue.

I did have issues with high temperature during the summer, but propping the lid open slightly (1/2") and running a fan during the hottest part of the day was all it took to solve.

I don't monitor pH or O2 enough to say what effect the lid had. I think your comment about the 1 open window is mostly correct. My lid isn't even close to air tight as I left off the plastic strip along the back (the one for cutting to fit tightly around equipment).

 

[Tamberav]

Acrylic lid would warp so skip that.

Glass lid is generally fine. It is just it will collect condensation and salt creep which is annoying. I have seen this cause spotlighting with some led's. The salt creep gets old fast.

Certain fish (wrasse in particular) may try and jump out and hit the glass lid and cause spinal damage.

I wouldn't worry about pH or CO2 in most cases as long as the tank has appropriate flow. I definitely wouldn't worry if you have a sump/skimmer.

 

[a.t.t.r]

I have done both lid and no lid. Temperature is much easier to maintain with no lid.
Auto top off is usually a much anyways and if you size it right or get one with some protections built in it is not a big deal flood wise. (Mine turns off if it goes longer then 10 seconds in any one refill assumes somethings wrong with sensor) and only holds a gallon at a time which is right at my overflow limit on a 20 gallon tank and last about a week.

 

[mdb_talon]

Finding hard scientific proof to show you is going to be a bit harder, but i can say without a doubt I agree with all of the "mantras" you listed other than maybe that it significantly changes the spectrum(i have no idea). Some of the others are going to vary by the rest of the tank setup also and other factors. Often mantras get that for good reason and are based off decades of experience of reefers. Sometimes they are wrong for sure, but often they are not.

Accumulation of CO2: As far as I understood that is entirely dependent on the CO2 levels inside the room and CO2 will balance with the CO2 outside the tank anyway.

I would highlight this comment because i see it a lot and think it is far from accurate. Co2 in the tank is always being produced/consumed, but how much of each obviously changes throughout the day. If a tank was naturally balanced with the room co2 however we would not see any ph swings at all(assuming alk remained the same). The reason almost everyone does see between a .2 and .4 swing daily is precisely because the tank is not in balance with the room co2. The more gas exchange you can provide then the closer you can get to a balance with co2 in the room. If there was pure sterile water in a tank then sure it would eventually become in balance. It is is not pure sterile water though and biological processes within the tank are always working to raise or lower co2

 

[KonradTO]

I have diffusers on my Radions, which effectively acts as a splash guard. While I'm sure the lights could tolerate a degree of saltwater, I'd prefer they didn't. Yes, a lid would minimize this - but I'm not sure I'm ok with the PAR, wavelength and other tradeoffs.

The biggest reason I don't like solid lids is the salt creep, restricted access and additional maintenance. Scraping my tank every week is bad enough already without adding to my chore list.

Skimmers and powerheads help to be sure, but if you have a closed sump and stand - airflow is still somewhat restricted. Nothing really beats a large open surface of water.

My point is that I am not entirely sure there is scientific evidence for your last statement. Is there anyone that measured dissolved oxygen levels with and without lids? Also, I had acrylic lid for a month now (so take this for what it is) but I had literally NO saltcreep. The water splashes on the lid and then drips down to the tank, I think humidity below the lid surface is too high for forming salt encrustations.

 

[KonradTO]

I have done both lid and no lid. Temperature is much easier to maintain with no lid.
Auto top off is usually a much anyways and if you size it right or get one with some protections built in it is not a big deal flood wise. (Mine turns off if it goes longer then 10 seconds in any one refill assumes somethings wrong with sensor) and only holds a gallon at a time which is right at my overflow limit on a 20 gallon tank and last about a week.

I have an acrylic lid on my 32.5g tank and to be honest I never top off, so I do not agree with the point that there is little difference. I also had many freshwater tanks both open and closed and the difference was huge, especially with higher water temperatures (I keep my tank at 26C, it should be 78F)

 

[KonradTO]

Finding hard scientific proof to show you is going to be a bit harder, but i can say without a doubt I agree with all of the "mantras" you listed other than maybe that it significantly changes the spectrum(i have no idea). Some of the others are going to vary by the rest of the tank setup also and other factors. Often mantras get that for good reason and are based off decades of experience of reefers. Sometimes they are wrong for sure, but often they are not.



I would highlight this comment because i see it a lot and think it is far from accurate. Co2 in the tank is always being produced/consumed, but how much of each obviously changes throughout the day. If a tank was naturally balanced with the room co2 however we would not see any ph swings at all(assuming alk remained the same). The reason almost everyone does see between a .2 and .4 swing daily is precisely because the tank is not in balance with the room co2. The more gas exchange you can provide then the closer you can get to a balance with co2 in the room. If there was pure sterile water in a tank then sure it would eventually become in balance. It is is not pure sterile water though and biological processes within the tank are always working to raise or lower co2

The swings are related to CO2 dissolved in water, correct me if I am wrong. On the other side CO2 below the lid is very similar to the one above, given that you are not in an airtight situation. Swings are there anyway because (as you say) of biological activity and as long as your kH is high enough will not be a problem. You also get pH swings in the ocean anyway, and that does not means that CO2 concentration in the atmosphere changes during the day/night.

 

[mdb_talon]

The swings are related to CO2 dissolved in water, correct me if I am wrong. On the other side CO2 below the lid is very similar to the one above, given that you are not in an airtight situation. Swings are there anyway because (as you say) of biological activity and as long as your kH is high enough will not be a problem. You also get pH swings in the ocean anyway, and that does not means that CO2 concentration in the atmosphere changes during the day/night.

I am not suggesting PH swings are bad. Just highlighting that the fact they exist conclusively tells us that the tank co2 is not in balance with the room co2 generally speaking (again assuming alk is stable and room co2 is not drastically fluctuating). I believe the suggestion they are in balance is a false premise. Anything that limits gas exchange is going to make it even harder for them to become in balance.

 

[blaxsun]

My point is that I am not entirely sure there is scientific evidence for your last statement. Is there anyone that measured dissolved oxygen levels with and without lids? Also, I had acrylic lid for a month now (so take this for what it is) but I had literally NO saltcreep. The water splashes on the lid and then drips down to the tank, I think humidity below the lid surface is too high for forming salt encrustations.

I'm not entirely sure there is, either - I just know what I have for air exchange inside my stand and it's significantly less than the open surface area of my tank. I don't have a closed lid to perform an ORP comparison, but I suspect it would be less with a lid (acrylic or otherwise).

 

[KonradTO]

With temperature I have to say that I agree. It is much more difficult to lower temperatures with a surface preventing cold air to cool the water surface. I am planning for this reason to use fans and I will change my tank temperature along the year, so I will have higher temperatures in summer (around 82F) opposed to winter (78F). in this way I have also a bit more action time in case heaters/cooling system fails.

 

[KonradTO]

I'm not entirely sure there is, either - I just know what I have for air exchange inside my stand and it's significantly less than the open surface area of my tank. I don't have a closed lid to perform an ORP comparison, but I suspect it would be less with a lid (acrylic or otherwise).

I think the point is always whether the difference is big enough for being biologically significant.. I would like to make the experiment myself at some point. I think oxygen concentration probes should be fairly easy to find on the market, and coupled with an arduino board it should be not too pricey..

 

[blaxsun]

Since you're either going to need an ATO or manually top up with a lid anyway, aren't we really talking about the added expense of a lid, fans, chiller, etc? (even excluding the additional maintenance and possible issues with oxygen exchange and lighting)

 

[KonradTO]

I am not suggesting PH swings are bad. Just highlighting that the fact they exist conclusively tells us that the tank co2 is not in balance with the room co2 generally speaking (again assuming alk is stable and room co2 is not drastically fluctuating). I believe the suggestion they are in balance is a false premise. Anything that limits gas exchange is going to make it even harder for them to become in balance.

Sure I get your point, saying that they are "in balance" is wrong. What I meant is that the air below the lid should have very similar CO2 concentration compared to the air above, Giving that there is enough air exchange. Also having macroalgae in the tank probably has a much bigger influence I suppose. I do not have a sump and I have my macros in the DT, therefore I cannot run the lights for the macros on night. I expect some pH swings, but for compensating I keep kH slightly higher than usual (10 dkH)

 

[KonradTO]

Since you're either going to need an ATO or manually top up with a lid anyway, aren't we really talking about the added expense of a lid, fans, chiller, etc? (even excluding the additional maintenance and possible issues with oxygen exchange and lighting)

Fans are pretty cheap if you DIY (4-5$). I never topped off (can you say that in english? XD) in a month. Actually I am trying to increase salinity a bit so I was planning to stop topping off for a while and salinity did not increase at all in 3 weeks since I started checking almost daily.
I understand your point, especially on the lid and chiller. I used a 5mm acrylic sheet for the lid which I paid something like 20$, but I am not considering that many people do not have the tools for making their own lid.
For the chiller there are as well ways to build it with less than 20$. In my case I live in Germany so the days when I really would need a chiller are too few for even considering it.. I had this summer the stock lid from my fluval flex, which has the light fixtures under the hood, and I was getting temperatures of around 84-85 F with the lights on, whithout lifting the lid. I guess it should be still within an acceptable range for most of tropical corals and fish.