Hi, This is my first post here

I have a 40 gallon tank, currently houses a pair of Albino Oscars and a pair of bala shark.

I got the Oscars 1.5 months back when they were 2.5 and 2 inches
right now they are 3 and 2.5 inches. I have never fed them feeders, just freeze dried tubeflex worms and fish pellets.

I had been out for a fortnight the past 2 weeks and mom was taking care of the fish, when i got back 2 days ago i noticed that one of the Oscars(the larger one) is breathing heavily, all the other fish including the smaller Oscar are doing fine and behaving as usual...

The larger Oscar opens his mouth quite wide for every breath he takes, and his gills too open quite a bit, as a result his whole skull appears to move back and forth . This behavior I had not seen before, and can not see in the other Oscar.

How ever both the oscars fins seem to have black spots on the fins below the belly and along the edges of the dorsal fins.
I'm not sure if he is eating or not, i'll look for that the next time i feed them.
I just made 40% water changes and i think i am going to do 20% daily changes till i find him back to normal. or i find out whats wrong with him.

Any ideas what might be wrong with him? I dont have any test kits as of now. The aquarium stores in my town do not keep them either because its sort of rural.

When i sit close to the tank, he comes and stays touching the glass the entire time i am there, I don't know the color of his gills though how do i see them? like do i have to fish him out and examine him or something?
I am very attached to my fish, would suck to have any die.
please advice.

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edit:
I have a 800l/h internal aerating filter

We need to know your water test reading Nitrate, Nitrite, Ammonia, PH, and temp.

And what is a fortnight? lol Your not from Texas are you?

A fortnight is 14 days( 2 weeks ), nope not texas - India actually.
I know the levels are the first line of diagnosis, but I haven't been able to get my hands on any, i think I'll get them on my next trip.

A fortnight is two weeks. Yes, without knowing what's your water conditions are, it's impossible to really help. It would be a little like going to the doctor and expecting him to diagnose a illness without taking a blood test. You need to get a test kit ASAP and then we can start helping you. Have you been using a water conditioner every time you change water?

water conditioner? never heard of it.

water conditioner is the anti-chlorine.
neutralize chlorine from tap water. it far more important than a test kit.
unless you have other better source of water

Yep. I ordered mine today an API Master Kit. Want to get the 'little un' in the kitchen just right, water wise, before I progress to the 'big un' lol

Prolonged exposure to chlorine will damage a fishes health so you must always treat the water you are putting into your fish tank with a water conditioner to remove the chlorine. Having said this, what part of the world are you? Maybe they don't put chlorine into your water. Better to be safe than sorry and check

Like i said i'm from india, i use ground water for the aquarium, don't think that has chlorine but the ground water might have other things -good or bad- i don't know. Anyway going out to bangalore(2hour drive) day after tomorrow, hope he can hang on till then.

40g is not enough room for 1 oscar let alone 2 oscars and 2 balas. You need to get a bigger tank or rehome your fish and get some fish that dont get so big

In the meantime I think your problems are related to poor water quality. As said get the test kit, do daily 25% water changes until you get your water tested. By the way only feed your fish what they will eat in 2mins

Aggreed on the need for a bigger tank.
What kind of filter do you have and whats the flowrate per hour?
btw welcome to the site!