there are many ways to catch fish.
the three most popular techniques are
1) cyanide
2) hand nets
3) mist or barrier nets.
for cyanide fishing, potassium or sodium cyanide is sprayed into caves, corals, water and any fish in direct contact will die. fish that are mildly contacted will be stunned and can then be easily collected. cyanide caught fishes rarely survive and almost all will die from organ failure etc.
for hand nets, a simple hand ned is used to catch fish swimming in mid water. often, a stick or jet of water is used to chase fish out of hiding and then the net will be used to scoop them up. this method requires alot of skills to execute and can usually only be done on small fishes like wrasse, damsels etc.
the mist or barrier net is my favourite technique. a large fence of netting is placed around some coral outcrop or on the seabed, and the fishes are chased into the giant net barrier where they will not be able to escape. then the divers will slowly catch them out from the net and place into their containers. this method can catch all kinds of fish and especially fast moving schooling fishes like tangs, haremic fishes like wrasses, and solitary/paired fishes like large angels and butterflies.