now, i am confused. yesterday, as i was feeding my oscar,
i saw some tiny insects inside my DIY plastic canister.
i am confused on how those small flying insect was able to
enter the air space inside the canister.

there is only one probability: the insects hatched from the
red worms of my substrate inside the canister. there is no other
way they can enter the canister except from inside hatching.

can any body give me your opinion!!!!!!!!!!!!!!!!!

I think we will have to do some research on this. However, I have got various old fish tanks in my back garden, some of them have filled up with rainwater. I happened to be in the back garden a few weeks ago and noticed lots of tiny little swimming objects in the water. This sounds the same as what you've got, I didn't examine them closely, but it does sound similar to what you have described. I'll have route around on the Internet later and see if I can find any more information

Thank you OFL.
i did check closely the insect inside the canister
this morning.... they are about 10 pcs more or less
similar to mosquitoes but without needle nose and very
thin bodies and legs. i also found out that those larvae
that i thought to be planaria have rolled with the algae
and dirt like a pupa of butterfly. they really hatched
inside the canister with an air space.

i will try to get magnified picture of the larvae, the pupa,
and the insects.

Bloodworms. Also known as mosquito larvae.