"Exotic species" may mean several things to different people, but I will take it as meaning the rarer and "uncommon." And I will confine myself to the soft water fish, most of what I will suggest being fish I have maintained and often spawned over the past 20 years. A 97 litre (approximately 25 gallons) is a good size for what I will suggest; back in the 1980's I had three of these with different biotopes, and I presently have a 29g which is probably my favourite of the seven current tanks in the fishroom.
You could do an Amazonian biotope; sand substrate (play sand is ideal, as it looks like the sands in many Amazonian streams), chunks of wood, branches, dried leaves (oak, maple, etc) on the substrate. You can forget lower plants and have only floating, which is very authentic, or have a few substrate-rooted plants like the chain swords plus floating. Depending upon the GH and KH of your soft water, the pH might easily lower to become more acidic which would be fine. As for fish, several of the species of pencilfish would be ideal: Nannostomus marginatus or N. mortenthaleri or N. rubocaudatus (these three are very closely related so I would only have one of them); N. eques and/or N. unifasciatus (these swim at an oblique angle which adds interest). Any of these can be combined. N. espei (this seems to be a very rare fish these days, but nice if you can find them). N. trifasciatus, but this one tends to be a bit nippy of surface fish which will impact my next suggestion. And that is hatchetfish, any of the species in Carnegiella will work as they are smaller and much more quiet so ideally suited to flooded forest aquascapes. Except for N. trifasciatus, the pencils mentioned previously are fine with hatchets. All of these species need a group, and I would aim for 10-12 of whichever.
Pencils tend to remain in the upper half, so for the lower half there are the corys; the "dwarf" species, but also some of the less often seen like C. guapore, C. similis, C. duplicareus, C. leucomelas, C. schwartzi, etc. A group of 6-7 of one of these, or you can have two or three species with 5 of each. Another interesting substrate fish that is rare is Characidium fasciatum; this is the most widely seen species when seen at all, but there are several others being discovered that may turn up. This is one of the most unique of the entire characin family, with its very inquisitive demeanour, and rather jerky swimming style. They do well in small groups; their only negative is that they are not always easy to wean onto prepared (non-live) foods, but if left in a smaller quarantine tank for several weeks they usually learn, though not always.
With all the wood, Farlowella vitatta is another good species, singly or a small group. While they get 5-6 inches, their body mass is so minimal they do not appear large, nor do they impact on the water quality the way a fish of this length would. My only negative would be plants; if you decided to have lots of lower plants, these fish with their continual rasping for algae can impact some plant leaves.
This is only one option, and a similar could be for SE Asian species. There are many "dwarf" rasbora species turning up now, and a few of the smallest gourami species can be included; these tend to be more delicate, so something of a challenge.
I have tanks along these lines presently, so I'll attach a photo of the Amazonian blackwater 29g and the SE Asian 33g as illustrative of the above.
Byron.
