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By not being fully cylced, i just added some water, some decor from my bro's tank, conditioner, and then fish. I had no idea about the whole nitrogen cycle in aquariums.

Please get yourself a API Test kit If you don't have one.
 
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Normally it takes 6-8 weeks to cycle a new tank from scratch.
If you add any fish right away, you should think of them as sacrificial, meaning you will be very lucky if they don't die within that 6-8 weeks.
Or, you can cycle by putting water in the tank, filters running "no" fish and add small amounts of the chemical ammonia daily until your tests reveal, the ammonia is immediately used by bacteria.
The test should read 0.00 ammonia, 0.00 nitrite, and some nitrate, once those results are there, that's when you add fish, not before. (6-8 weeks)
Since you did the former, you must do daily large water changes to dilute the ammonia the fish are putting out.
And that ammonia is enough to rob most fishes immune system of any viability, making the tank a bastion for even non- infectious bacteria to become killers.
 
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Normally it takes 6-8 weeks to cycle a new tank from scratch.
If you add any fish right away, you should think of them as sacrificial, meaning you will be very lucky if they don't die within that 6-8 weeks.
Or, you can cycle by putting water in the tank, filters running "no" fish and add small amounts of the chemical ammonia daily until your tests reveal, the ammonia is immediately used by bacteria.
The test should read 0.00 ammonia, 0.00 nitrite, and some nitrate, once those results are there, that's when you add fish, not before. (6-8 weeks)
Since you did the former, you must do daily large water changes to dilute the ammonia the fish are putting out.
And that ammonia is enough to rob most fishes immune system of any viability, making the tank a bastion for even non- infectious bacteria to become killers.

ok, i am already doing daily water changes, but i usually do only 20%. im currently doing a water change now, and im increasing it to 50% water change.
 
Sorry about the loss of your Oscar we all make mistakes in the beginning of this hobby, but always try to do some research or ask questions that’s what always helped me I’m still learning things to this day.
 
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Do you know anyone else near you that in the hobby?

Cause if so, it would be a great benefit to your fish, if you could snag some cycled media from one of their tanks. If you don't want to lose the rest of your fish then this is the safest way to go at this moment.

I agree with all others, with no way to test your water params, then a FAT water change every day is almost mandatory (because at this point we know you're not cycled)

The very first time i ever cleaned a fish tank I killed like 7 or 8 fish. I knew nothing about the nitrogen cycle that occurs in an aquarium. I replaced the filter pad, washed the sand out real good in hot tap water, and changed 100% of the water. So I killed off basically 100% of the bacteria that was eating their ammonia.

Theres countless videos on YouTube about cycling a new tank so I would definitely watch some to get an idea of what is going on in your tank. Its really very simple

1st step- Fish pee and poop all day which releases Ammonia into the tank.

2nd step- To combat this Ammonia build-up there must be bacteria that eat Ammonia,
and their waste product is Nitrites.

3rd step- To combat this Nitrite build-up from the bacteria, a second type of bacteria is
present to turn these Nitrites into Nitrates

4th step- To combat the Nitrate build-up you do water changes. Lots and lots of water
changes.

I really think you will realize soon that a 40 gallon tank is not near enough for even one oscar, and they really do great on their own. We (humans) seem to think that they need buddies.

Best of luck to you and the fish. Go get some cycled media from someone, its the best bet.
 
Do you know anyone else near you that in the hobby?

Cause if so, it would be a great benefit to your fish, if you could snag some cycled media from one of their tanks. If you don't want to lose the rest of your fish then this is the safest way to go at this moment.

I agree with all others, with no way to test your water params, then a FAT water change every day is almost mandatory (because at this point we know you're not cycled)

The very first time i ever cleaned a fish tank I killed like 7 or 8 fish. I knew nothing about the nitrogen cycle that occurs in an aquarium. I replaced the filter pad, washed the sand out real good in hot tap water, and changed 100% of the water. So I killed off basically 100% of the bacteria that was eating their ammonia.

Theres countless videos on YouTube about cycling a new tank so I would definitely watch some to get an idea of what is going on in your tank. Its really very simple

1st step- Fish pee and poop all day which releases Ammonia into the tank.

2nd step- To combat this Ammonia build-up there must be bacteria that eat Ammonia,
and their waste product is Nitrites.

3rd step- To combat this Nitrite build-up from the bacteria, a second type of bacteria is
present to turn these Nitrites into Nitrates

4th step- To combat the Nitrate build-up you do water changes. Lots and lots of water
changes.

I really think you will realize soon that a 40 gallon tank is not near enough for even one oscar, and they really do great on their own. We (humans) seem to think that they need buddies.

Best of luck to you and the fish. Go get some cycled media from someone, its the best bet.

alright. Thank you all so much for all the advice. I just have two questions:
1. What is a FAT water change?
2. The only person near me that keeps fish is my brother. And he just has a low tec tank with goldfish, mollies, sword tails, guppies, and plecos. And he has a hang on the back filter that has one of those mesh bags, not a sponge. So how do i obtain cycled media from his tank?
 
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