Is the 50ppm 'limit' something to do with the maximum amounts fish can cope with?
This subject has come <"up a few times on the forum">, but because you can't search for "CO2", you have to search for terms like <"hypercapnia"> and <"Bohr Root"> effect........I have found it difficult to get a definitive safe limit for CO2 toxicity to freshwater aquarium fish. The best I've been able to find was something I discovered recently related to Rainbow Trout. This is the reference below and note it's only an abstract but it contains useful information nevertheless. Here it is:
Take a look at the date of this paper. This research was carried out no less than forty years ago!
I've also found the graph (that I couldn't find the other day) <"showing the effects of pressure"> on gas (in this case O2) solubility.I've just found that coke link, which says "0.14mol CO2 in 1000mL".
I'm not a CO2 user, so I don't have any practical experience of keeping fish at enhanced CO2 levels. It isn't an avenue I'm ever likely to go down, but if I were I would be aiming for a lot less than 30 ppm CO2.
Just keep away from fish from cool, flowing water really. So definitely not Hillstream Loaches.So if we were to set an upper limit of, say 20 ppm CO2, which fish species would be suitable in this environment?
Bit of a funny one really, fish like Pterygoplichthys gibbiceps are very tolerant of low oxygen levels and can take in oxygen from gulped air, against that some of the rheophilic Chaetosoma spp. require really high oxygen levels. Air gulping is an ancestral trait in both Corydoras and Loricariids, derived from their mutual ancestor.Clearly, plecos are not candidates.
It is exactly as @Alex C says, they are directly related because the haemoglobin transports both CO2 and O2.I was talking about CO2 but you are talking about fish that are tolerant of low O2 levels. Am I missing a trick?
That’s correct,.....but at that point you have a nutrient deficiency. Or you could say I have extremely high co2, very high ferts but your lights are too weak, now you have a lighting deficiency. It’s a circle, everything needs balanced. But to answer the question, no high Co2 doesn’t inhibit plants growth. I believe I mentioned this before but there is a point of diminishing returns.Assuming that high amount of CO2 will make one of other nutrients quickly depleted, we can say that excess CO2 may inhibit plant growth.