In the wild they are most often reported as living in massive shoals (1000’s not 100’s) so would 6 individuals really provide any more “security” than 4, both numbers are far far fewer than a thousand
I do think that having similar coloured corydoras makes a difference to otocinclus comfort level
This PFK article is worth a read, especially the part where author mentions collecting trips where otos are few in number & appear as by-catches, he further mentions their possible mimetic behaviour with similarly coloured corydoras
This is something I’ve observed in a planted sale tank containing a good number of otos & pygmy corys - as the otos are sold off, they appear more & more “paired” with the similarly coloured & sized cory’s (the otos always sell quickly as they are much less available than some years ago, their price has also increased substantially)
Seriously Fish O macrospilus profile - otocinclus species are often incorrectly reported on fish sale sheets, this seems to be the species that is most common in local shops (though I don’t think I’ve ever see a farm list with this species name
)
Given the small size, it’s likely 4-6 individuals would do fine in a 45cm tank (if sufficient food)
Bigger Otocinclus species, I’d be inclined to agree with a 60cm minimum tank length
More Otos always seem more active “happier” than fewer Otos, if sufficient food I’d always keep at least 6-10 and suspect 20 would be much better
But in my experience I can never seem to wean them onto vegetables (maybe this says something about the algae level in my tanks
which I swear is minimal ... ) so I don’t like to buy more than 5-6
Funnily enough they seem to love Hikari frozen bloodworms & get very excited when I feed those, definitely scarfing down several bw (I usually only feed bw twice a week as I rotate foods)
Note that all local shop otocinclus are wild caught fish (I don’t know if there is much trade in domestic otocinclus)
I’ve transferred my group between a 60cm x 45cm tank and a 90cm x 45cm tank, I don’t observe much change in behaviour but both tanks are 55cm tall and 45cm deep, I believe fish are much more comfortable in tanks with more depth front to back than standard US10 & 20 gallon tanks provide
(BUT I just got an ADA 60P
as it’s such a lovely tank, and has fantastic proportions (back to Amano discussions) -
I added 25 juvenile M. kubotai (today) though I’m not sure how long they’ll stay there, likely just for quarantine (no other fish in the 60P))
If fish give every appearance of thriving, it’s hard to argue that a bigger tank or more fish is “better” ...
I’m convinced that most fish would prefer to live in water with less than 20ppm nitrates (keep altums or Sphaerichthys species and it’s obvious when nitrates exceed 10 - 15ppm) but try and convince EI method aquarists to lean dose