Using fish as a CO2 indicator.

[Dennis Wong]

This is a short note for folks that use fish as a CO2 indicator. i.e. Some folks turn up the CO2 to the point that fish surface, then turn it down slightly. The key question is - what levels do this indicate?

From my testing of the various fish available in my tanks, both fish and shrimp are more resistant to CO2 than most folks think. It may also vary depending on the Oxygen levels in the tank at the same period.

The minimum levels I have seen that to induce a change in behaviour in the fish I have is around 60ppm of CO2. Above 60ppm of CO2, CRS shrimp seem slightly more agitated (but no obvious surfacing) and Oto catfish gather at higher areas of the aquarium. However, above 60ppm Cardinal tetras and Amaya pencilfish still show no symptoms at all.

These fish show no reactions at all at 50ppm+ CO2 : White tip tetra, White cloud mountain minnows, Gold tetras, Cardinal tetras, Hyphessobrycon 'Cherry red', Amaya pencilfish, Ruby tetra, Hyphessobrycon Myrmex, Crystal red tetra, Bloody mary shrimp and CRS continue to breed. So virtually all the fish that I have on hand are unaffected by 50+ppm of CO2.

My guess is that for folks using basic tetras rising to surface as a CO2 gauge, you are probably 70-80ppm+ of CO2 to cause them to surface. From what I have seen in my own data set and shop's data set, 30+ppm of CO2 doesn't cause any species of fish to rise to the surface. I have been told Discus is sensitive to CO2, but I have not yet tested this.

 

[Sb1415]

Thanks I have noticed this in my tank. On an additional note on fish type and their behavior.

My guppy and platy (especially the guppy) will act lethargic, as if sleeping when co2 gets high, but my tetras, Cory and Otos will go on with their day. I just got a Hanna co2 kit so can’t comment on the co2 level when this happens but if I were to make a guess I would say between 40 - 50 ppm.

 

[hamfist]

Thats really good info Dennis. In truth I think knowledge of ideal CO2 use in aquaria is still almost in its infancy at the moment. Its great to have people active in the hobby actually knowing how to experiment and actually testing with decent equipment. Many thanks. I am very much in a "CO2 phase" at the moment as I implement it on quite a challenging tank so I'm all ears for this kind of stuff.

 

[Yugang]

The title of this thread should be food for thought for us hobbyists. Not criticising Dennis, as for some reason it has become common practice to think along these lines.

Some want to grow plants that need a lot of CO2 or farm them commercially. If you choose this, just grow plants without exposing livestock.

The far majority of plants thrive at 30 ppm, so perhaps we can learn to adjust our community fish tank methods for a healthy balance of plant and livestock wellbeing.

 

[gjcarew]

I've noticed fish sometimes behave more scared at higher co2 levels. Like they will hide in the plants

 

[snowballs99]

This also depends how well u can keep the surface clear to maintain oxygen levels.

If you run a HOB filter and attempt to run CO2 that high you'll get really bad surface film that will choke out the fish.

 

[Naturescapes_Rocco]

And to complicate everything more, fish health is a factor!
I find fish that have internal parasites (white stringy poop, swollen or sunken bellies), or who arrive very sick from the distributor, are often the first to start gasping at the surface, while healthy fish of the same species are often fine.

I still recommend a Hanna CO2 test kit as the best method for testing CO2 levels!

 

[Unexpected]

I've noticed fish sometimes behave more scared at higher co2 levels. Like they will hide in the plants

My blood fin tetras behave this way.

 

[Mr.Shenanagins]

I still recommend a Hanna CO2 test kit as the best method for testing CO2 levels!

My next purchase, where did you buy yours from? Everywhere wants to gouge you on shipping

 

[Naturescapes_Rocco]

My next purchase, where did you buy yours from? Everywhere wants to gouge you on shipping

I bought direct from Hanna! They must be getting popular because they keep selling out.

 

[Dennis Wong]

This also depends how well u can keep the surface clear to maintain oxygen levels.

If you run a HOB filter and attempt to run CO2 that high you'll get really bad surface film that will choke out the fish.

Yep. I don't consider HOBs real filters.... lol...

 

[hamfist]

It gets me thinking about how many of these thoughts/opinions are actually based on implied or assumed CO2 levels originally derived from pH measurement (or even reactor surface area). Dennis' experiments have at least been with an insanely expensive CO2 meter. THis gives them extra weight with me. It's nice to see folks quote CO2 ppm from an actual CO2 meter or assay of some description - but its not the norm, yet. Even on a good forum such as this one its much more usual to see CO2 ppm levels quoted derived from other measurements and assumptions about KH, pH, complete degassing etc. There are many potential errors there, thus observations based on such assessments of CO2 carry much less weight in my eyes. It seems that many folks who have started to buy CO2 test kits such as the Hanna one are realising that their actual CO2 levels are rather higher than they had been assuming.
I truly don't know you guys well enough to know who measures with what and I don't want to make any individual assumptions but I will say that the first thing I look for in a CO2 level thread these days is what device or assay the person is measuring actual CO2 level with. This can, and should, make a real difference to how we view any evidence/observations derived from them.