The tank should be full at all times with liquids nutritious to SCP-XXXX (raw sewage etc. – this may be sourced from the facility’s own water system if necessary).
The second half of the line reads sort of awkwardly and not particularly clinically. I'd change it to "The tank should be full at all times with liquids nutritious to SCP-XXXX, such as raw sewage" with "this may be sourced from the facility’s own water system if necessary" behind a footnote.
The captive specimen has seven segments,
The adjective use here seems weird, I'd cut out "captive" and, if you so choose, change "specimen" to something else, like "subject".
nine (9)
Most articles don't do this anymore. You can keep it, but shrug
though the maximum number of segments possible has yet to be estimated.
This line sticks out, mainly because it's a discrepancy. The previous line, "The captive specimen has seven segments", is fairly affirmative, but this line implies that there can be more. In such a case, I'd change the initial line to "The subject currently has seven segments".
with each stomach serving as a kind of filter.
"kind of" should probably be cut.
Following a month of submersion in a tank full of sewage, secretions from the rearmost rectum of SCP-XXXX were analysed.
This part is strange to put here, since the Description should be purely describing the SCP. You can put this in a footnote or something, but its inclusion is sort of meh.
During testing, SCP-XXXX made no attempt to latch onto or otherwise attack personnel.
Same thing here. If you're set on including this, my advice would be "Detached segments do not display aggression or hostility" or something along that nature.
The entire paragraph before the investigation section is iffy in terms of wording and readability.
For one, I'd group the entire paragraph under "Discovery" or something similar, since it's no longer about the description.
were already working a case in Japan; several members of the populace had suspiciously died,
This, uh, seems a tad too on the nose. I'd change this to more subtle and concise wording.
"Several Foundation operatives were present in Japan to contain SCP-XXXX, following a case wherein several members of the populace had expired" and so on.
As for the investigation, I feel like it should really be put under an Addendum (i.e. Addendum XXXX-1 - Investigation) or something, just having "investigation" as a category on its own isn't that great.
The product was seized under the premise of a drugs raid, but the Foundation must prepare for the possibility that SCP-XXXX is already gestating in persons internationally.
I'll get to this a bit more in my General Feedback, but this is basically leaving nothing to the imagination.