If the fish won't eat them, the best way is just to net it OR let them grow and become adults and leave the tank. Mosquitos lay eggs in still or stagnant water, so just make sure you get more water movement.
What is strange is that I have a fairly powerful external power filter on this tank and it does not seem to affect the larvae. I need to find a way to stop that tank from being a mosquito breeding station because of chikungunya (mosquito carried disease).
I would believe it is mosquito. Because you have a large external filter does not mean you have a lot of flow. My 30g tank with a 900gph stream maker has a lot of flow, 30 turnovers per hour. My 75 SPS coral tank with 2,400GPH has 32 turnovers per hour. That's a lot of flow. As filters get dirty they slow down. Get yourself a powerhead and put it near the surface of the water. It will prevent it from happening again. As to your current problem I'd say guppies but your oscar should eat them in seconds. I have a small pond int eh yard that I keep guppies in to eat the mosquitos.
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