What Species Would Be Best?

[nadajdui]

My 33 gallon currently houses 1 BN Pleco and 1 keyhold cichlid. In another week or so 7 diamond tetras are going to be going in there from the quarantine tank they are currently in. I am overfiltered with Aquaclear 70 and Aquaclear 50 HOB filters.
Ph is around 6.8, very soft water.
I believe I have room for something else, and I was wondering if anyone had any suggestions on oddball fish, as I think it would be neat to have something unusual.
Thanks.

Oh, sorry, Tank is as follows:
L 36" W 12" H 18"

 

[Dragonscales]

Perhaps a topdweller like an ABF would add some lacking movement up the top of the tank.

 

[nadajdui]

What's an ABF?

Oh, nm lol. Brainfart. African Butterfly Fish. I really like those, but my Diamond Tetras nip at everything, even the fake plants, so I don't think that would be a good idea.
Would killifish work maybe?