Why not go for a 5 foot tank?
this way you have room to spare for some other, more lively fish
the tiny Ageneiosus are really nice (Ive seen Marcs' ) but they're not exactly the worlds' liveliest fish.
if I'd use one word to describe them "torpid" would do nicely
in a 5 footer you'd have room to spare and you could add Hoplosternums, Dianema, Brochis, Opsodoras, mediumsized highbuild Characins like Poptella or Gymnocorymbus or something; none of these competes with the Ageneiosus for food (they don't eat small fish) and their lively antics are useful; might give the Ageneiosus a feeling of "safety". dithers in fact.
a speciestank with 5 small Ageneiosus of the more sleepy species will be something like a speciestank for Bunoccephalus......
it's your tank, mate; but I'd like to see movement in a tank now and then
they're quite different from the larger Ageneiosus magoi Marc had a long time ago.
now there's a really active cruisin' Ageneiosus.
Marc had only three, but I still remember that tank. they were visible and active, even with the lights on.
awesome fish. if the tapwater here was unsuitable for Tanganyika or Madagascar and suitable for Amazonian fish (unfortunately it isn't) that would be the fish I'd build a tank for. with some Doradids and the larger Callichthyidae and some goodsised Characins.