you cannot catch a quilfish in leaf green. you must trade for it from another Pokemon game.
Edit:OKay when you beat the elite4 you will be able to go to the other islands okay go to island 6 and it is common to find one so far ive caught like 3 so i can trade with emerald Good LUCK
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you can't catch qwilfish in emerald. the only gba Pokemon game that I know has qwilfish is fire red. However to the answer stated above you can catch Qwilfish in Pokemon Colloseum, purify it, and then trade it to Emerald via. a GNC/GBA connector cable.
You can not catch it in leafgreen,but in firered you can.
Nowhere no hoenn Pokemon can be captured in leafgreen.
You can find it by using a Super Rod on Iron Island.
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