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Sol, Cyan here is the best thing to do (i've never had luck with the 8-shot or lightnight etc)
Come into the room, make sure all the fish hatch, hang out on the bottom. They will chase you down, run around the outside, but hand the walls tight. So that each corner they take, the pack of fish gets tighter.
Eventually, you want a single 'sprite'/pack of the fish. Now whip around a corner (bot left works best) and they come into the middle room, still low though. So what you want to happen, is the single fish, is stuck on the outer wall, trying to swim to you (you are at his level) then you can beat the two oysters on your level freely. Next, take out the fish, swim up, and just go back and forht hitting the oyters, they shouldn't be able to produce new fish.