Apart from the obvious water quality and oxygen level when having a lot of fish, is the actual overcrowding in itself stressful? Meaning things like swim room. Ie are those 200 guppys in the 40cm tank in fish shops severely stressed even if the water quality and ability to breath is OK. Also meaning are fish likely to bring diseases if kept in tanks of an extreme amount in such a small space?
There are two aspects to overcrowding. The usually more obvious one is physical space, and the second (which is frequently misunderstood) is water quality. Though each has distinct impacts, they also have a related combined impact. And yes, all of this is highly stressful to fish.
If fish are crowded, the water quality is never "good" because it just cannot be good under such conditions. We cannot eveen begin to test for much of this; ammonia, nitrite and nitrate may be zero, but that is only one very small aspect of water quality. Oxygen and CO2 is significant but again not thee major issue. All fish communicate through chemical signals; phermones are read by others in the species, and allomones are read by other species. Fish continually release these chemicals, which is one major reason for significant water changes because it is the only way to remove them. Fish can be under serious aggressive threat just from these chemicals even without actual physical aggression. Initially we do not see external evidence, but at some point it will become so severe it will be evident, and by then it is usually too late for the fish. The physiological and psychological damage caused by
severe stress is irreversible.
Fish carry many diseases, naturally. They develop ways to handle them, naturally. But stress at any level begins weaking the fish, especially their immune system--this is true of all animals, including humans, which is why reducing stress is a major component of fighting off most diseases. This causes further stress, and it just multiplies until the fish can no longer cope and it dies. The immediate cause may be "x" or "y" disease, but the actual underlying cause of the death was stress. Stress causes 95% of all aquarium fish disease.
Each species of fish "expects" a very specific environment, and this is part of the fish's DNA. The more the conditions we provide differ from those in the wild habitat of the fish species, the more likely thee fishes are to be stressed and susceptible to disease.
Fish in store tannks are in every case (with a very few exceptions) being forced to exist in highly unsatisfactory conditions in terms of what they "expect." But most fish can manage to tolerate the bad for a limited period, and it usually has no lasting effect. But this is not always the case. Normal/general store tank conditions should never be regarded as adequate because they never are.
