Fish IDs from the office tank project:
5 Serpae Tetra
1 Oto Catfish, I am surprised it's alive given the high nitrate levels.
2 False Rummy Nose Tetra
1 Pepper Cory
The Common Pleco who has outgrown this tank a long time ago.
Surprisingly they are not a bad mix as far as water parameters go... it's a little hard and a little cold but no worse than other "working" environments.
The easiest would be to turn this into a species tank and up the Serpae Tetra to 15... pretty much the max load for the tank assuming lots of plants. Given those numbers I think that the larger shoal would be much more visually appealing than three or even four almost too small groups of fish... the chance for inter species compatibility goes down for every new group added and I think that the smaller each group is and the smaller the tank size the faster this drops.
I might consider starting with new fish altogether (store will probably take them all) but would love input as to what might be a good single species fish, colourful and/or shoaling tendencies. I have gone through and looked at pretty much every fish here and, with a single species, there are lots to choose from and few that I am familiar with.
Tank is a 20 gallon, medium to hard water (although I need to test the source as this may be artificially hard, 15dGH) neutral pH, temperature must be over 70F.
Other fish considerations, cannot be livebearing, plant eating or large enough to limit the numbers any lower than 10. Not looking for fish that do best as pairs or singles and nothing that is finicky to keep or nocturnal. It would be nice if they were small enough to be able to add shrimp to the tank... but that is secondary. They cannot be neon tetras.