air stones when CO2 and lights are off

[Werwa]

I have a 120 gallon tank heavily planted with CO2 and hi-tech lighting. I just added 6 otocinclus cats. I had 6 months ago and only one survived and seems to be thriving. My LFS advised I add an airstone to increase the O2 when the plants are respiring. Never heard of this before. Interested in other opinions. Thanks everyone!

 

[_Maq_]

It is a well-known fact that all organisms in the tank (incl. plants) respire, mostly oxygen, and during night the oxygen level may become dangerously low. However, it very much depends on many variables, so our tanks differ a lot in this. Personally, I always prefer O2 to CO2, but many others think differently.
Besides, if injecting CO2, some more factors play a role. Diurnal pH fluctuations (which are a reverse of natural ones), and often mentioned stability in CO2 level as an important factor.
So your question can be easily answered in principle, but quite difficult to decide in your particular situation.

 

[Werwa]

Thanks for that response. Would there be any harm in using an airstone at night with CO2 off?

 

[Simon Cole]

I am now minded to think that there are good options for <suitable air stones> and a matching pump.
Please could you estimate:

a) average flow across the water surface (e.g. cm or inches per second)​

b) water surface area (length multiplied by width)​

...and please feel free to include information like:

c) estimated carbon dioxide concentrations and measurements​

d) temperature​

e) filters, skimmers, etc.​

f) full tank photograph​

...do ask if you need any guidance. I would be very happy to give my opinion

 

[Werwa]

I will try to give you some of the info you requested.
Water surface: 1,152 square inches (48" by 12")
temp: 78 degrees
filters: Oase Biotherm 350 (350 g/hr) & Marineland Magniflow 360 (360 g/hr)
CO2: hard for me to tell, but looks like 3 bubbles per second with internal diffuser and spray bar

4 pearl gouramies
7 otocinclus cats
20 Brilliant Rasboras

 

[Simon Cole]

Thank you for your reply.
This next bit is all just boring calculations, so feel free to skim through:

Your aquarium length and width: 121.92 cm by 30.48 cm.
Your aquarium surface area for air exchange: 3716.12 cm squared.
Your filters are advertised to achieve a combined flow of: 3227.73 litres per hour.
Observations:

Plenty of ripples of the surface indicating good water surface agitation, spray bar outlet, so perhaps the velocity of you water at the surface averages at around 8 cm per second.​

Rippling will slightly increase the water surface area of your aquarium.​

Plants and fish (and microbes etc.) will respire carbon dioxide at night, and you aquarium is fairly well-planted and stocked. Looks great by the way ​

The velocity of most air stone bubbles is 28 to 30 cm per second, but this possibly speeds up when there are lots of them (35 cm per second seems a reasonable assumption for now).
The difference in velocity between bubbles provided by an air stone and your aquarium surface water I would estimate to be at a ratio of around 4.38 to 1.
40 bubbles that are 1 mm in diameter will add 12.5 cm squared of surface area for air exchange at static velocity.
11892 bubbles that are 1 mm in diameter will add 3716.12 cm squared of surface area for air exchange at static velocity, matching the surface area of your aquarium.
Assuming there is a increased boundary layer diffusion generated by the velocity of air stone bubbles, which is assumed to be at a ratio 4.38 times faster than flow at the aquarium water surface, then 2715 bubbles that are 1 mm in diameter would probably achieve the same rate of air exchange.
The volume of a 1 mm diameter bubble: 0.5235987755983 mm cubed.
The volume of 2715 bubbles at 1 mm diameter: 1,421.570675749385 mm cubed, which is equal to 0.00142157 litres.
Error noted:

Noting that you mentioned that your 120 gallon tank is 12 inches wide and 48 inches long, this implies that it is 48 inches deep. I will assume that the figures above need to be corrected to: 48 (long) by 24 (wide) by 24 (height) in inches. Accordingly...​

Assuming that the air stone bubbles rise at 35 cm per second for 50 cm before they reach the water surface, then there would need to be air pump injecting 5430 bubbles at 1 mm diameter at a rate of: 0.0019902036 litres per second in order to match the assumed air exchange achieved by the surface of your aquarium water.
This works out as only 7.16473296 litres per hour.
Many smaller aquarium air pumps state they will easily put out 50 litres per hour at 0.18 mbar pressure, but that is cheeky marketing because it equates to less than 5cm water depth. The pressure at 50 cm water depth is in fact approximately 49 mbar. Anybody who has messed around with low pressure carbon dioxide enrichment systems will know what I mean. So in order to get air stone bubbles matching the air exchange of the surface you your aquarium, aim for 5430 bubbles at 1 mm diameter, and try to find a pump that can deliver those results at the given the depth of your aquarium... something that is broadly rated for an aquarium of your size. I do think that one of the air stones pictured <here> will achieve enough bubbles to match your surface air exchange.

Edited: There is however likely to be a <marked decrease> in dissolved oxygen levels when bubbles are at greater depths. This means that at face value you may seek even greater rates of air injection and bubbles than previously thought. The main role of air stones is to enable water column movement from the bottom of your tank to the surface. If that is not a factor because you already have high aquarium water surface flow and agitation, then a safe bet would be to double these figures and go for two of those larger air stones.

There are also a few points worth mentioning:

• Oily films and biofilms on the surface of you aquarium water will reduce surface air exchange and potentially decrease dissolved oxygen (DO) levels. Surface skimmers can sometimes help.
• Your spray bar will be providing considerably enhanced levels of dissolved oxygen anyway.
• Decreasing temperature slightly can improve dissolved oxygen (DO) levels (if the species present do not mind).
• Toxicity associated with excessive dissolved carbon dioxide enrichment is rather common.
• You can estimate your dissolved carbon dioxide by doing a 24 hour pH profile. Many of us also rely upon carbon dioxide drop checkers as visual checks.
• Adjusting carbon dioxide enrichment can avoid the problems associated with carbon dioxide toxicity.
• Many people turn-off their carbon dioxide enrichment a few minutes or hours before the lights go out, and ramp it up again before the lights turn on in the morning (using a solenoid timer).
• As you might expect, air stones increase air exchange and reduce carbon dioxide levels accordingly.
• Air stones are harmless to aquatic life in terms of toxicity, they are very safe, but I have noticed that they may make certain fish more timid when powerful ones are running (certain fish are nocturnal feeders).
• Never attempt to inject pure oxygen from pressurised cylinders because it is remarkably toxic, from what I have been told.
• Getting robust measurements of dissolved oxygen levels usually requires a rather expensive meter and probe, but we do have members who have worked with them in the past if you go down this route.
• Fish can of course die for a variety of different reasons.

Conclusion:
My views on aquarium air pumps have been gradually evolving over time. I am minded to think that your LFS offered some rather good advice, not just in terms of dissolved oxygen levels, but also reducing the risks associated with carbon dioxide. I would try to pick an air pump and air stone that delivers what you want it to achieve, and be mindful of other causes of fish mortality.

 

[Werwa]

Thank you for such a robust response. Very much appreciated.

 

[Werwa]

Just noticed an error on my part. The tank is actually 24" by 48". Not sure if that changes your recommendation, but looks like I will add an airstone at night to restore O2. Thanks.