# Tank cover, good or bad for CO2?



## tiger15 (9 Dec 2018)

Its inevitable that some injected CO2 will gas out of the surface.  Will a glass cover help capture the escaped CO2 in the airspace above the water allowing better redistribution and diffusion back to the water?  Has there been experiments comparing CO2 injection efficiency between open top and close top set ups?


----------



## foxfish (9 Dec 2018)

I think a loose  fitting top would help conserve the C02 but you would have to seal the top and allow the air space to pressurise to re gain any gas bubbles reaching the surface.
Think of a carbonated bottle of water, as long as the lid is sealed the C02 can’t escape, if you take off the lid and shake the bottle the gas will escape.


----------



## Edvet (9 Dec 2018)

tiger15 said:


> some injected CO2 will gas out of the surface


I think it''s more then some, i think we waste over 95% compared to what the plants use.


----------



## tiger15 (9 Dec 2018)

foxfish said:


> I think a loose  fitting top would help conserve the C02 but you would have to seal the top and allow the air space to pressurise to re gain any gas bubbles reaching the surface.
> Think of a carbonated bottle of water, as long as the lid is sealed the C02 can’t escape, if you take off the lid and shake the bottle the gas will escape.


I don’t think it is practical to have tight top seal as openings are needed for equipent inlets and outlets nor is it beneficial to shut out O2 exchange.  Nevertheless, an increase in partial pressure of CO2 in the air space should decrease outgassing and promote redistribution and redesolving of CO2.  I’m wondering if someone has already looked into it and made comparison.


----------



## Barbara Turner (9 Dec 2018)

The new house I'm living in has pretty poor ventilation and is showing signs of damp. Just bought two big dehumidifiers.  
I'm also going to be fitting my lovely braceless tank with a glass cover to try and reduce water loss. The design has 3mm clearance so won't be a tight fit. I  have also designed it to allow my lilly pipe to go through. My other half has basically said get a cover or get rid of the tank..

Im wondering if I'm going to have to put a bubbler on at night or if the plants will be sufficient to provide enough o2  for the fish.


----------



## cvx195 (9 Dec 2018)

Is there a chance that this could go a little to far keeping to much co2 and causing there to be oxygen deficency in the tank?


----------



## tiger15 (9 Dec 2018)

cvx195 said:


> Is there a chance that this could go a little to far keeping to much co2 and causing there to be oxygen deficency in the tank?


If there is O2 deficiency, you’ll notice fish gasping for air similar to CO2 poisoning, except that it occurs off photo period.



Barbara Turner said:


> I'm also going to be fitting my lovely braceless tank with a glass cover to try and reduce water loss.
> 
> Im wondering if I'm going to have to put a bubbler on at night or if the plants will be sufficient to provide enough o2  for the fish.


Close top will reduce water loss for sure, so will it reduce CO2 loss.  Don’t count on plants to replenish O2 as plants don’t produce O2 and consume O2 at night.  Your filtration/circulation system is your primary oxygenator provided that the airspace above the tank is not completely sealed off.  It also depends on the type of filtration.  HBO and sump system are open system to the atmosphere, so they are good oxygenator.  Canister is a closed system, not so good, and it relies on the return flow to break the surface for oxygenation.


----------



## foxfish (10 Dec 2018)

I have often wondered what would happen if I covered the top of my tank with cling film.. when the gas is on, would the film stretch tight or even expand up into a dome?


----------



## Edvet (10 Dec 2018)

No, not enough pressure from the gas. Co2 will collect ( CO2 being heavier than air )under it but not cause pressure, cling film might be porous for CO2 btw.


----------



## Oldguy (10 Dec 2018)

Barbara Turner said:


> get a cove


I use a cover glass to reduce condensation and have never had CO2 problems. If you inject CO2, I assume you use a timer and solenoid.


----------



## rebel (10 Dec 2018)

These loose covers will have no effect. Inject away!


----------



## dw1305 (10 Dec 2018)

Hi all, 





foxfish said:


> I have often wondered what would happen if I covered the top of my tank with cling film.. when the gas is on, would the film stretch tight or even expand up into a dome?


No, it would just go into solution, it is the carbonated drink scenario. 

The amount of CO2 that dissolves into the water is defined by <"Henry's Law">, that is dependent upon pressure and temperature.  CO2 is actually a very soluble gas, you just don't get much dissolved in the water normally because there isn't much in the atmosphere (about <"405 ppm">). 

If you create a CO2 rich atmosphere, with the cling film, then a *lot more *CO2 will go into solution, from <https://www.engineeringtoolbox.com/gases-solubility-water-d_1148.html>. Potentially, at 20oC, you can get over 1500ppm CO2 dissolved in the water (this would require the gas above the water to be 100% CO2).

* 


 *
cheers Darrel


----------



## rebel (10 Dec 2018)

I think the cling film will just dislodge and start to leak. It may not be that great for fish that may need to breath air though.


----------



## Edvet (10 Dec 2018)

dw1305 said:


> carbonated drink scenario.


I read they take out ALL the air/oxygen first then add CO2, but i thought they would add it pressurized to get more in, while we use atmospheric pressure to add CO2.


----------



## dw1305 (10 Dec 2018)

Hi all, 





Edvet said:


> I read they take out ALL the air/oxygen first then add CO2, but i thought they would add it pressurized to get more in, while we use atmospheric pressure to add CO2.


There is an answer to that one <"here">, which I think says it is  pressurised to  about three and a half atmospheres at 4oC, and contains about  0.14mol CO2. 

CO2 has a RMM of 12+(16*2), so 0.14 molar CO2 = 6.16g in 1 litre or 6160 ppm CO2. 

Which makes it not surprising that Amano found that sparkling water aided plant growth, and why flat coke doesn't taste quite the same (you've had a major change in pH) as bubbly coke.

cheers Darrel


----------



## Oldguy (10 Dec 2018)

dw1305 said:


> sparkling water aided plant growth


When I was between CO2 regulators I used to add supermarket cheapo fizzy water at 17p for 2 litres. The cheaper the brand the lower the mineral content of the water. You can get upside down fish if you are heavy handed. Did that once about 30yrs ago with Soda Stream carbonated tap water. All recovered. You can also anesthetize slugs and snails with the stuff.


----------



## tiger15 (10 Dec 2018)

Global warming has winners and losers.  There are reports of higher crop production due to elevated CO2 in fields not Impacted by water constraint.  It’s a popular practice by marijuana growers  to inject CO2 in green house to increase production.  I heard that 3000 ppm is the safe limit for CO2 in green house before affecting people.

Returning to the original post, why are we spending so much effort to design a perfect CO2 reactor when a rough one with a tight tank top can probably achieve the same outcome; in other words, using the tank top airspace as a reactor.


----------



## rebel (11 Dec 2018)

tiger15 said:


> why are we spending so much effort to design a perfect CO2 reactor when a rough one with a tight tank top can probably achieve the same outcome; in other words, using the tank top airspace as a reactor


That's because you need the tank surface for O2 exchange. With your technique, you will need to inject O2 separately. 

Also you'd need to make sure there is a very tight seal around the tank, which would not be easy or aesthetically pleasing.


----------



## tiger15 (11 Dec 2018)

rebel said:


> That's because you need the tank surface for O2 exchange. With your technique, you will need to inject O2 separately.
> 
> Also you'd need to make sure there is a very tight seal around the tank, which would not be easy or aesthetically pleasing.



Delivering O2 separately to tank water is quite easy.  Just run an air stone. 

I understand that it is infeasible to have a completely tight top as there are openings for equipment inlet and outlet, and one has to open the lid to feed fish and plants periodically.  A non tight cover will not stop O2 exchange just like our insulated house will not suffocate us.  Due to better insulation, the newer homes have worse indoor air quality than older homes due to accumulation of leaks from household chemicals. 

But a partially tight cover still will increase CO2 partial pressure in the tank airspace similar to our indoor air having higher CO2 concentration than outdoor.  The question I have is whether there is quantification on how much it will help deliver and distribute CO2 to tank water.  This subject has not been discussed in my search.


----------



## dw1305 (11 Dec 2018)

Hi all, 





tiger15 said:


> But a partial tight cover nevertheless will increase CO2 partial pressure in the tank airspace similar to our indoor air containing higher CO2 concentration than outside.


I think that is where the danger lies, it would be more efficient in terms of preserving the CO2 you've added, but you run the very real risk of asphyxiating your fish.

I'm not a CO2 user, but if I was I'd want a system with a large gas exchange surface area. You would use more gas, but small increases in the CO2 addition rate would be much less likely to cause fish death.   





tiger15 said:


> There are reports of higher crop production due to elevated CO2 in fields not Impacted by water constraint. It’s a popular practice ......


Same with tomatoes, growers have used additional CO2 for at least 30 years. The cost benefit analysis depends on the value of the crop, and the cost of the CO2, but about 1200 ppm CO2 is somewhere near the "sweet spot". <"Prolonged exposure"> to 5000 ppm CO2 is pretty bad for you.

The modelling of changes in crop growth with increasing CO2 levels is an area with a lot of <"scientific research interest">.

An interesting recent finding is that the large areas of irrigated corn (Maize (_Zea mays_)) production in the American Mid-west has <"both increased rain-fall and reduced summer high temperatures">. Maize is a C4 plant, so CO2 levels don't have much effect on plant growth (for physiological reasons). The climatic change is thought to be due to the increased transpiration of the Maize compared to the native grasses, and is reliant on the artesian water extracted from bore-holes. When the water runs out  the best guess is that temperatures will start to climb and the rain-fall reduce.

The issues with anthropogenic CO2 production are likely to be due to the climatic warming and the effects on weather patterns. 

cheers Darrel


----------



## zozo (11 Dec 2018)

tiger15 said:


> There are reports of higher crop production due to elevated CO2



That is actualy something oldschool, the first reports kinda surfaced already in winter times with the use of Natural gas and Propane greenhouse heaters. They are very popular in the crop growing industry.

Since burning propane = C3H8 + 5O2 --> *3 CO2* + 4H2O
And natural gas = CH4 + 2 O2 -> *CO2* + 2 H2O

Very effective, back in the day with those household Gas water heaters with pilot flame, combusting in the room it was installed. These rooms grow plants like champions.  So you realy don't need a massive burner to notice effect, for a small room a simple pilot flame is already enough to encrease production.


----------



## tiger15 (11 Dec 2018)

I will not suggest anyone to convert his open top set up to cover top for the sake of CO2 conservation. Any gain in CO2 conservation will be offset by loss of light intensity through glass, which can be as much as 50%.   For me and others who keep jumpers, having a tank cover is needed and not a choice.  To compensate for reduced light intensity and distribution, I tied submersible LEDs to tank top rims.

Other winners of global warming are plankton and whales.  There is likelyhood the northern pathway will open up all seasons allowing penetration of light and expansion of plankton habitat and the whales that feed on them.


----------



## Oldguy (11 Dec 2018)

tiger15 said:


> using the tank top airspace as a reactor.


Early methods of introducing CO2 into aquariums did use a bell jar method. The area under the glass bracing strip at the rear of the tank can be converted into a linear bell jar by gluing glass ends & front so that they are below the water line and bubbling CO2 gas into this space. I considered it but went for the plastic tube with a helical insert. CO2 was bubbled up from the bottom and rose against a down current of water from a canister filter. The idea was that the gas had dissolved before it escaped through a grill at the top. Still have the kit, by Dennerle, now in storage.


----------



## rebel (11 Dec 2018)

tiger15 said:


> Delivering O2 separately to tank water is quite easy. Just run an air stone.


Wouldn't that break your seal on the top? Also the mechanism of O2 delivery from an air stone is not from the air itself but by surface agitation. Remember O2 is less soluble than CO2.



tiger15 said:


> Due to better insulation, the newer homes have worse indoor air quality than older homes due to accumulation of leaks from household chemicals.


This is not a good analogy that's relevant to this discussion.

I understand your concept though. I agree it hasn't been 'tested'. Time for you to test it.


----------



## tiger15 (12 Dec 2018)

rebel said:


> Wouldn't that break your seal on the top? Also the mechanism of O2 delivery from an air stone is not from the air itself but by surface agitation. Remember O2 is less soluble than CO2.
> 
> 
> This is not a good analogy that's relevant to this discussion.
> ...



True, oxygenation occurs predominantly by breaking the surface than delivered by the air bubbles.  But air bubbles,  with 20% O2, is captured in the air space in contact with the breaking surface, same mechanism.

I wish I could test the concept myself, but my set up won’t allow.  If I remove the tank cover, the lights that sit on top will fall in, and fish will jump out.  Test is feasible and easy for set ups with over hanging lights.


----------



## Oldguy (13 Dec 2018)

As far as I know still in use in the UK.


----------



## Sick1166 (17 Dec 2018)

I am running a glass cover on 60u for several weeks at a time for evaporation loss while traveling for work,
no difference on co2 results with top on so as others say keep on pumping


----------

