Where do tropical fish wholesalers get their fish from?

Hi, I recently purchase some fish from a wholesaler. I know that regular fish shops purchase from them, but I was wondering where does wholesalers get their fish from? and why doesn’t fish shops just buy from them instead of the wholesalers?

Fish for consumption:
I know of an independent fish shop that buys straight from the source, but further inland they’d probably buy from a wholesaler because the wholesaler might provide a delivery service whereas the source might not.

Ornamental fish:
I know that for marine aquariums almost all of the fish are fished straight from the reef and sold. Same reason as above though for aquarium fish shops not buying straight from the source (unless the wholesaler does the fishing as well): transportation of the fish is probably provided by the wholesaler as opposed to the people who go out and catch the fish. To be honest, at a quick glance I think tropical marine fish are a bit unethical to buy/encourage the trade of by the looks of this article: http://www.buzzle.com/editorials/8-5-2006-104634.asp

Freshwater aquarium fish are different though (presumably because freshwater is easier to maintain compared to sea water); apparently in the US most of them are bred in man made ponds in Florida.

‘Relative to other aquaculture ponds, tropical
fish ponds are very small, averaging 25′×75′,
with a maximum depth of about 6 feet’

I’d imagine the wholesaler will have the fish imported to a certain state or country from the source and then individual aquarium fish shops will buy from their nearest wholesaler. The aquarium shop won’t have to import a small number of fish from a long distance (costing alot per fish) as opposed to having the wholesaler import lots of fish from a long distance (not as much cost per fish delivered) and it probably working out cheaper for the aquarium fish shop and the consumer.