QUESTION: Hello Ron, I have a 2nd 75 gallon aquarium that has been established for a decade now. After I sold the last couple of fancy goldfish,I decided to get a lone Oscar. He is 2"-3" long.

I know they create a lot of waste, grow large, etc but I'd like to know what kind of maintenance schedule I'll have when he's getting bigger, then full grown. He was bought at a chain store so hopefully he'll grow!

I have 2 Eheim canister filters, a 2213 & 2215. I believe the 2 together rates for 150 gallons. I know nitrates will have to be watched & do water changes. I just need to know how many & how much.  

I've been told so many different things (on an Oscar Forum) that I actually put the fish in a bag (w/water of course) & was about to drive 50 miles to the store where I got him to get my money back.

I drove a couple of miles & came back w/him, thinking this is crazy lol! I really have become attached to him already. I hope the advice is good.


ANSWER: Hi Paula,
 You will be glad you kept him. They are very interesting and personable fish.  There are a few key things with oscars.  First, never feed him feeder goldfish.  Feeder goldfish carry all sorts of parasites which rapidly and easily infect and kill oscars. If you follow this advice, you will be much happier.  Second, ideally you should change 25% of the water once a week, every week. It is okay to skip a week every now and then, but if you keep up the water changes, and do not overfeed your little guy, he will be with you for a long time.  One of the real joys of keeping a fish like an oscar is that you will quickly see that he can easily recognize you as distinct from any other person.  

-- Ron Coleman
  Cichlid Research Home Page <>

---------- FOLLOW-UP ----------

QUESTION: Thanks Ron but on the forum I had since joined said to do a 50%-75% water change one-two times per week, otherwise Nitrates will cause Hole In the Head disease!

Also, they said do not keep 2 Oscars in a 75 gallon, which is something I had wanted to do.

Right now my well established tank is going through a mini cycle after we got rid of our turtle. Ammonia is at .5 ppm & Nitrates are between 40-80 ppm!! I'm doing large water changes, took out all the sand substrate, which is what I was told to do. I'm hoping this will end soon! I do not overfeed him. Btw, I couldn't feed a live creature to another one so no worries there.

Would you be kind enough to answer all of my concerns?


ANSWER: Hi Paula,
 You can always do more water changes than 25% every week, but 75% twice a week seems excessive. Part of the problem is overfeeding the fish. Fish do not need a lot of food -- look at the size of the stomach!  But, if you put a lot of food in, you are going to get a lot of waste out. A fish should NEVER be fat no matter how much he begs for food (oscars quickly learn to train their owners into giving them more and more food).  

  Lots of people have kept 2 oscars in a 75 gallon tank for years. What you likely cannot do is to have one oscar in that tank, then later try to introduce a second oscar.  I can pretty much guarantee that that will not work out well. If it were me, I would get four little ones and start them out together then see which two form a pair later on.  

  You should put the sand back in. The sand serves two important functions. First, it allows the oscars to dig, something that they like to do.  Second, it provides a ton of surface area for good bacteria to live on.  You only want about an inch deep of it but I would definitely put it back in.

-- Ron C.
  Cichlid Research Home Page <>

---------- FOLLOW-UP ----------

QUESTION: Thanks Ron, you made my day! One thing is wouldn't the 2 outsiders get attacked before I could find them a home? I only have a spare 29 gallon to put them in, if that happens.

Couldn't I just get 1 more Oscar since I've only had this one for 3 weeks?

Hi Paula,
 Yes, the two outsiders would eventually have to go elsewhere.  

 You could just get a second one, but it is impossible to sex oscars despite what some people tell you, so if you started with four, you have much better odds of getting at least one boy and one girl, which is the combo most likely to live successfully together.   

-- Ron C.
  Cichlid Research Home Page <>


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Ron Coleman


I am an expert on cichlid fishes, particularly New World cichlids. My broader expertise includes the behavior, ecology and reproduction of fishes in general. (I am NOT an expert on Goldfish). Please do not use abbreviations, such as "my GT has a swollen eye" because I don't know what a "GT" is. The more clearly you can explain your question, the better chance I have of understanding what it is that you seek. I keep fishes both as a scientist and as a hobbyist and I currently maintain about 140 aquariums.


I am an Associate Professor at the California State University, Sacramento in the Department of Biological Sciences, and I run a website, called the Cichlid Research Home Page . I also write for many popular aquarist magazines, and I was editor of Cichlid News magazine for several years. I am a scientist and I spend my time teaching fish biology, ecology, behavior and evolution and doing research on the reproductive biology of fishes, particularly cichlids. I do research in the laboratory and in Costa Rica, Nicaragua and Mexico. My main interest is understanding the evolution of parental care in fishes. I am interested in encouraging greater public awareness, understanding and participation in science.

Cichlid News, Tropical Fish Hobbyist, Freshwater and Marine Aquarium, Science, Behavioural Ecology and Sociobiology, Copeia, Canadian Journal of Zoology, Trends in Ecology and Evolution, AUK, Environmental Biology of Fishes

PhD (Toronto, 1993) MSc (Simon Fraser, 1986) BSc (British Columbia, 1983)

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