Black Ghost Knife Fish Growth

[NeilFishman]

Hi everyone,

Pretty new to the fish keeping community and I recently got a black knife ghost fish. The aquarium shop owner said that I'd eventually need a bigger tank, but wasn't specific. As Im sure everyone in this community knows they need a lot of space as they grow; I'm reading 100 gallons minimum.
At what size should I consider upgrading to a 100 gallon tank? My BKGF is currently ~6" in a 40 gallon tank.
Also in a 40 gallon tank, are there any fish species recommended as a tank mate?
Though I feel I should wait to introduce one, until I get an answer to my original question.
Please advise. Any information is useful, he seems content at the moment with plenty of hiding spots, good water parameters and he's not shy. I managed to tong feed him day 3! But I'd like to provide him with a good environment to live his life when he is fully grown...

Thanks!

 

[Fishman Dave]

Whilst they are very manouverable and can hence probably live in smaller tanks than some other fish. Just to give context. I upgraded mine from a 100 community to a 180 at around 8” as he was looking cramped and out of place. Then from the 180 to the pond at around 14”.

 

[duanes]

I keep 2 eight inch Panamaian knives in a 180 gal with cichlids.
Since they are nocturnal, unless I am up after the lights go out, (or you keep them under a red spectrum bulbs), its a little hard to judge how much space they actually need, when they are most active. I hardly ever see them moving around at all, they spend most daylight hours hiding in crevices in sucken water logged wood, so on the surface, it appears they don´t need much space, but.........

The video above, is when they were caught in Rio Terable, a couple months ago.

 

[benzjamin13]

I currently have a BGK in a 100g. I haven't measured it in quite some time, but it was in a 20g long grow out/quarantine tank for about a year. I would guess it to be between 6"-8" at the moment. IME, they don't grow very fast, so in essence, I had my BGK in the 20g until it was 6" before moving it to the 100g. I don't remember exactly when I moved it, but it was either late 2025 to early 2026. Picked it up when it was around 2.5". My previous BGK grew to about 11" and housed in a 120g for 6 years. It started off as a 2.5" fish as well in a 75g. I believe I kept it there for probably the first 2 years of having it and grew to about 8" in the 75g. It was housed in a community tank setting with Serpae Tetras, Siamese Algae Eaters, and Plecos. While no fish went missing, after a year went by in the 75g, I noticed 1/4 of my Tetras were missing an eye. I assumed the BGK might've been taking a nip at them. Once removing the BGK, no more missing eyeballs.