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Q: How much space does a Knifefish need?

February 10, 2010 | By Alexander N. | 2 answers | Expired: 1512 days ago

How large of an aquarium does a Black Ghost Knifefish need?

I have a 37-gallon tank with 10 fish, all of which seem very satisfied with the amount of room they have and their niche in the aquarium. However, there are some that are expected to live several years and grow significantly in that time.

The resident I'm particularly concerned about is my Black Ghost Knifefish. I bought it (does anyone know how to tell their sex? dimorphism of any kind?) about two months ago and it's grown to about five inches. While that's not a lot (it is both uninterested in, and incapable of, bothering the other fish in the tank) they apparently reach over 12 so I'll likely need an upgrade eventually to accommodate it along with my other fish (two gouramis, four danios, two catfish).

The current tank is a great size for our living room so I don't want to go too large, but it's more important to make sure I'm taking good care of my fish.The gallon-per-inch rule would mean about 50-60 gallons, but the Knifefish alone may need that much. Does anyone have experience in keeping such large fish and their space needs?

Readers' Answers (2)
Michele Z.
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Feb 11, 2010

If you are currently keeping the knifefish in the SAME tank as your other fish (gouramis, danios, and catfish), I'd highly recommend that you IMMEDIATELY segregate the knifefish--it should NOT be in the same tank as the other fish, unless you want your other fish to be eaten by the knifefish (which I presume you do not). As the knifefish grows, it will undoubtedly eat smaller fish IF they are available.

I don't know that much about large fish, but I once had a catfish that grew to about 16". The young man at the pet store told me it was a Cory Catfish, but as the months passed and the catfish grew and grew, I eventually came to realize that the store employee knew nothing about what he was selling me! I initially had the catfish in a 10 gallon aquarium with several other small fish, then moved the catfish to a 30-gallon tank (when it was about 5"). Catfish (and some other fish) can limit their growth to "fit" in the space they have, but that really isn't fair to the fish.

I'd say you are correct in your estimate of how large of a tank you need. (A 55-gallon tank for the knifefish ALONE should suffice.) But PLEASE separate this fish from the others ASAP.

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Michele Z.
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Feb 11, 2010

Also, you might need to keep a large 55-gallon tank on the ground floor (e.g., in a gameroom, if you have one) since the weight of a full tank is very heavy and therefore poses a risk of your floor collapsing.

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