It's such a complex issue, and so much of the information available seems to be quite dated.
It is also part of human nature to lean towards the information that backs up what they think, e.g.
@Sue example of "forgetting" that particular information relates to pencil-shaped fish.
Looking at the links that
@fcmf posted, my favourite bit from the PFK article was
"So stocking densities are an oversimplification of something more complex?
Yes. Exactly. The actual amount of fish a tank can support is determined by its carrying capacity, but calculating this is complicated and beyond the average fishkeeper."There is so much to take into account when considering the welfare of fish, and so much of the information available can be contradictory. As
@Hampalong has noted, use the rules as a starting point for research.
My knowledge and experience is what I would describe as "limited", and so far I think I've been quite lucky to have the experience and information available here to make sense of information I find elsewhere. It has always been encouraged to stock based on adult fish size, and not to stock to full capacity, whilst also taking into account the nature of the fish.
It could be my imagination, but now there also seems to be more of a leaning towards what I think of as "creating environments" for fish. In general, people have become more aware of animals and their suffering, in all aspects of life. Limiting any suffering and/or harm, caring for the environment, and trying to do the right thing is encouraged. With regards to fish, magazines and online articles show biotopes, and the creation of environments that mimic those found in the wild. On top of that local fish shops provide quite a selection of plants, decor, etc. and on-line shopping can pretty much get you anything. With a bit of artistic licence (can a fish tell the difference between a natural cave, or half a coconut shell covered in moss? and does it care?) we can provide our fishy friends with what they need, within the confines of a tank. Fingers crossed we are keeping them happy, healthy, and entertained.