I don’t think an adult Oscar is going to be bothered with tiny mosquito larvae but it’s worth trying the not feeding them approach. Even my little fish can go ten days without feeding so an Oscar should have no trouble going a week without food.
As far as tank mates that will hunt down the mosquito I can’t think of anything that would not be eaten by the Oscar.
Have you tried netting them at the surface with a fish net? Each mosquito must come up for air every few minutes.
Do you have a water test kit ?
What are the nitrates levels?
Are you changing some tank water every week?
I do not understand how mosquitoes make the water unfit for fish? Many fish use mosquitoes as a food source in nature and in aquariums.
Mosquito have two stages of larvae which I am sure you realise? The first long wriggling stage is used in my country as food for many fish. The second stage is a totally different looking creature looking like the letter e . This black ball has a harder shell and is not recommended for small fish as the shell are block the stomach.
The two adult Oscars would not eat mosquito larvae. Sorry I don’t know of any fish that would eat mosquito but not be eaten by an Oscar.
I suggest the issue you are having is the 20% water changes with two Oscars in a 75 gallon. Try a weekly 50% change.
It’s not recommended to change 100% of the water in a fish tank.
Do you have chlorine in your tap water?
What are you adding hot water to ?
Do you have a test kit to determine if you have ammonia or nitrates?
I have not used this product myself.
Mosquito Dunks is sold in the US and in Australia.
To answer your question.
I have no experience with tiger barbs but would assume they would eat mosquito larvae?
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