So your just regurgitating information you found off some site? lol. BGK will not get to 50cm overnight,it will take years after it slows down drastically after 30cm. It's not an active riverine fish anyway.Since I'm here, I'd like to point out that the footprint of the 852 liter wouldn't be quite big enough for an adult 50 cm ghost knife (so maybe it's a good thing that that site was a scam!).
Going by the 6x the length and 1.5x the width rules of thumb from Practical Fishkeeping and Seriously Fish, you'd need a tank of at least 300x75 cm for the knifefish long term (even though the weather loaches I proposed would be fine in the 852).
Those 2 sites are known for outrageous tank sizes.
Extract from PFK. com-
Can I keep knives?
For a fishkeeper liking movement and interaction, knifefish are not the way forward.
Tank size should be chosen according to the size of species kept. For the common Black ghost (Apteronotus albifrons), a fish that can reach 50cm/20”, an aquarium of around 3m/10’ should be considered for a fully grown adult.