Philip,
The arapaima in Thailand were imported and live in lakes, so totally different to their natural environment where they live in rivers and then when the rains come and the rivers flood they move into the forest. So the wild ones tend to be a lot less fat from what i have seen from photos - would love to go for in South America.
It is almost impossible to weigh arapaima due to the blood vessel running the length of their back which bursts if the fish is bent round at all, leading to its death, hence the estimated weight.
My friend Stuart Gillham who has a lake in Thailand was sending details of some of his fish to IGFA when he was enquiring about what would be needed to claim the record , and from the length measurements he gave they didn't believe his weight estimations, then he gave them the girths and they changed their mind, saying that the wild fish are totally different and a new scale would be needed for the Thai fish.
I can't remember how big Mekong catfish go, i believe they grow very large in the wild, but they are plankton feeders and i believe will only take bait once they've been in a water for a while and get used to it, other than accidentally inhaling your rig whilst they are feeding.
Siamese carp also grow to huge sizes, in excess of 450 lb i believe, but the real big ones rarely come up for sale and are hugely expensive, so lake owners generally don't get hold of them - a big Siamese carp is a status symbol in Thailand (much like the Japanese with Koi) so the rich businessmen buy the biggest that they can to keep in their garden pond (lake).