Do you know the article you read this on? Is any of it permanent health issues? I am not denying it, I am simply interested in reading on it. I am a fish over plants guy myself so I am concerned about the well being of my fish. I suppose it can't be too bad (depending on co2 concentration, fish species, duration of exposure, water chemistry, etc.) given the amount of people who keep fish in co2 injected tanks with no reports of long-term ill effects (besides immediate co2 gassing of fish), but I am still curious, if there are any scientific studies showcasing the effects that abnormal levels of co2 can cause of fish. I will try looking up some studies myself, but would be interested in seeing the source you read.
Here are some articles I have read (you can find many more with Google):
Here you have an extensive literature on CO2 toxicity for aquatic organisms at the end of the article.
2) Chronic CO2 exposure markedly increases the incidence of cataracts in juvenile Atlantic cod Gadus morhua L.
I had a conversation with Damian Moran (one of the scientists), and he recommended me to not exceed 10-15 ppm CO2 if I want be safe with my freshwater fish.
5) The effect of carbon dioxide on growth and metabolism in juvenile turbot Scophthalmus maximus L.
Also note that not all ill effects may be visible with the naked eye. I compare the effects of elevated CO2 levels with air pollution. For example, in many China cities (like Beijng) the air pollution exceeds the permitted values up to 30 times in some parameters. See the picture:
The AQI values should not exceed 30 over 24h.
Now, do you think that there are no people living there in these areas? And do they have children? Of course, they have. And do these children go outside and play there with other children? Sure! So at first glance you see no ill effects on people. But when you look at the statistics of respiratory diseases, heart diseases, cancer, sickness rate, mortality, lifetime period etc. then all these factors show you very clearly that the pollution has a direct impact on the health of the people.
With CO2 or any other nutrient pollutant it is very similar. I don't say this to freak you out, but it is true that many hobbyists overlook these things. If the fish won't die or have no clear visible symptoms they think it is OK. But if we would make a dissection or study it in controlled environment (like scientists do), we may come to different conclusions.
We adopted to use CO2 gas in our tanks without seeking to find out first if it may have any negative effect on our critters (or even plants).
=> We even stoped to use our horse sense ("What CO2 levels are common in tropical rivers with the lush growth of all species of aquatic plants?" So why do we use more?).
We adopted to use Estimative Index fertilizing method without seeking to find out first if it may have any negative effect on our critters.
We adopted to use different gadgets (like Twinstar) without ... it's always the same. Some crazy guy tells us that it's safe, and we just believe him (without any verification).