I have a baby Oscar Paco and koi in a 55 gallon tank. how fast will they grow?

Answers:
Whew - these are two fish that do not adapt (size wise to their surroundings). The Paco, however, will stay small if you practically starve him to death (and they can live for years on very, very little food). If you feed them a lot (and both of them will nearly eat all the time if you feed them constantly), they will outgrow a 55 gallon tank quickly (say == about 6 months, maybe less). If the Paco starts growing much faster than the koi, he will start taking little bites out of him! Pacos can be very aggressive because they are like an American Freshwater bass.

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Very. You'll soon be able to drop a line in the tank and go fishing.

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Odd combination. Since Koi is more of a pond fish It really should have more space, and Oscars need at least a 70 gallon by themselves because of how much waste they produce, they will start to grow in a few months to a year. I suggest when they get bigger you should consider getting a bigger tank. That will prevent crowding and could prevent your koi from being aggressively attacked in the future.

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Is your oscar named Paco, or are you asking about a Pacu?

Too fast. Koi do not belong in anything less than a 125-gallon tank, and even that should only be temporary housing. They belong in large ponds.

Oscars require at least 55-gallons for each fish because of potential size and how much waste they produce.

I understand both fish are babies, but they really aren't compatible, as koi are coldwater fish and oscars are tropical fish. Just out of curiosity, what is the normal temperature that you keep your tank at? And what does your pH test at?

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the bigger the tank the bigger the fish will grow.Oscar's tend to get very aggressive as they get bigger.and the Koi is a very passive fish, there is gonna be a problem I'm afraid.just thought I'd let you know in advance..good luck and have fun watching how big the fish get.
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Basic set up - If you are starting out with baby Oscars, you can start with a 30 gallon aquarium. However, go into it knowing you will be upgrading in 6 months or less. I would recommend to anyone who wishes to own a pair of Oscars to start with a 55 gallon tank. Two Oscars can grow from a young age to adult size and exist in a 55 gallon tank. As long as you only have the two you may not need to upgrade your tank size. Keep in mind, that a 55 gallon tank would be the minimum tank size that I would recommend as the size of Oscars will vary. It is very possible that your Oscars, when reaching adult size, will outgrow even a 55 gallon tank. A 70 gallon would be the ideal size to start with as your Oscars will grow fast, and have ample room to swim.