I believe the gradual change in understanding has come from the world of university scientists and others who of course have slowly continued to add to the scientific literature and the understanding of fish, how they function and the nature of stresses to them. The big problem for humans is that many things that may bother or threaten another species are simply not "observable" by ordinary means. We humans can be happily pointing to our fish and telling someone how nice they look just swimming there in the sparkling water, when to our sudden shock we watch them drop dead. Our worlds are very different. High ammonia, high nitrite(NO2) and insufficient oxygen are all quite invisible to the naked eye.
One complication of one-off anecdotes is that different species tolerate and react differently to different levels of different toxins. Likewise, different individuals within a species vary quite a bit too. So, while a scientist might find a species of river-bottom catfish that can live just fine in 1000ppm ammonia levels, a stock of new tetras of one sort or another might succumb to being introduced to a tank with 0.4ppm or so of ammonia concentration.
The modern message is that ammonia, even in tiny amounts, causes permanent gill damage that can result in less than ideal function in the animal, which in some species may result in shortened lifespan or death. Nitrite(NO2) is dangerous in a different way. Its oxygen atoms are able to occupy the same position that O2 would on the hemoglobin protein on fish red blood cells. The nitrite alters the charge state of the iron (Fe3+ state rather than the Fe2+ state in the Heme group) and brings on a methemoglobin situation. The red blood cells under the microscope look like they turn to brown mush and the blood can look a weird sort of chocolatey blue. In a sense, its suffocation but the first result ends up being nerve and brain damage due to the high oxygen need in those tissues.
My intent is just to explain what I understand about the stuff, not to stress you more. We all live in less than ideal environments on the earth and its obviously going to take decades for average hobby practices to slowly become more humane.
Anyway, enough with the sciency stuff. The practical outcomes are that now that we understand Fishless Cycling, there's no longer a need to expose our fish at all to the poisons that need to be present to create the biofilter, they can just be introduced after the filter has been qualified! Its such a simple and complete solution, I think that's why many have adopted it, even some of the old-timers like myself and OM47 here in the beginners section.
Likewise, our methods of dealing with Fish-In Cycling Situations have improved and fewer fish are lost in these unfortunate circumstances. What's been found is that adjusting the size and frequency of your water changes to keep both ammonia and nitrite(NO2) between zero ppm and the lowest level on typical liquid test kits (eg. 0.25ppm on API and 0.30 on Nutrafin kits for instance) is a level that will be protective of nearly all species and is doable, if difficult, for the hobbyist.
Often what we find is that doing a couple of very large gravel-clean-water-changes initially (after learning this information) will bring the levels down in the right range and then the needed continuing changes will not be as difficult. One of the first questions we get is usually whether the bacteria will be starved of their ammonia "food" by these large water changes, but the answer is than no, they can develop quite nicely. Full size colonies of bacteria are maintained quite nicely when our kits read zero ppm for both ammonia and nitrite, so those same colonies are able to develop when our kits can't measure much.
I think it will give you peace of mind to work on your percentages and frequencies of the water changes such that you bring those 0.50 and 1.0ppm nitrite levels down between zero and 0.25ppm to ensure the goldfish are sustaining nerve damage. Another problem some of our stocking experts will probably notice is that about 20G for the first fancy and 10G per additional one is usually recommended.
~~waterdrop~~
