"Having considered the form and function of fish and the chemistry of their watery environment, our attention now turns to the practical aspects of providing healthy conditions for pond and aquarium fish. The first step is to gather as much information as possible about the natural environment and behavioural characteristics of the fishes you wish to keep. The more the conditions you provide differ from those in the wild, the more likely the fishes are to be stressed and susceptible to disease. A little forethought in planning and subsequent regular maintenance can go a long way towards keeping your fishes healthy."
[Chris Andrews, Adrian Exell, Neville Carrington, Peter Burgess, The Manual of Fish Health, p. 67]
Fish do not learn how to feed, they are born with the knowledge imbedded in their DNA. The genus Geophagus is Greek for earth eater, because of their manner of eating. The fish expect to find their food this way. Preventing this is inhumane.
The majority of aquarium fish do not eat other fish in their habitats. Examination of stomach contents makes this abundantly clear. In the habitat, the possible predator and prey never come into contact, but within the confines of an aquarium things can be different.
@GaryE did a thread a few weeks back about how it is the fault of the aquarist through not providing these things that cause bullies and such.
Fish experience stress from every negative factor. Stress is responsible for over 90% of all disease in aquarium fish. All stress is negative because if the stressor is not removed, it continues. Biology Online defines stress:
"The sum of the biological reactions to any adverse stimulus—physical, mental or emotional, internal or external—that tends to disturb the organisms homeostasis; should these compensating reactions be inadequate or inappropriate, they may lead to disorders."
The effects of stress on fish are very complicated physiologically, and are often subtle. There may or may not be external signs discernible to us—it can continue for weeks and even months, sometimes up to the point when the fish just suddenly dies. The reasons for this are involved.
Adrenaline released during the stress response increases blood flow to the gills to provide for the increased oxygen demands of stress. The release of adrenaline into the blood stream elevates the heart rate, blood flow and blood pressure. This increases the volume of blood in vessels contained within the gills, increasing the surface area of the gills to help the fish absorb more oxygen from the water. The elevated blood flow allows increased oxygen uptake for respiration but also increases the permeability of the gills to water and ions. This is what is known as the osmorespiratory compromise (Folmar & Dickhoff, 1980; Mazeaud et al., 1977). In freshwater fish, this increases water influx and ion losses. This is more critical in small fish than larger due to the gill surface to body mass ratio (Bartelme, 2004).
Short-term stress will cause an increase in heart rate, blood pressure, and respiration as described in the preceding paragraph. The fish can only maintain these altered states for a short and finite period of time before they will either adapt or (more often) the stress will become chronic. During this initial stage the fish may look and act relatively normal, but it is depleting energy reserves because of the extra physiological requirements placed upon it. At the chronic stage the hormone cortisol is released, which is responsible for many of the negative health effects associated with stress.
