Plecs and other bottom feeders aren't, as far as I know, compatible with the bi-orb media so I hope the pebbles you have added are smooth and cover the bottom completely?
Assuming that the bi-orb 105 holds 105 litres or 28 gallons of water, under normal circumstances (in a rectangular tank) you would be looking at aprox 28inches of adult fish. With the bi-orb having less surface area however, you are better off staying on the conservative side of this, say 20inches, which probably means you are already there or slightly over. How about adding some shrimp, or snails? If you really want something else, a couple of platies would probably max it out, they would add some colour and they are peaceful and use every part of the tank so are a good choice for communities. They are also quite short lived so if your BN is a baby, by the time it reaches 4 inches or so the platies will have lived out their two year life cycle.
I know the tanks look cool, but they have to be so conservatively stocked I would always go for a regular 3ft tank over the bi-orb design. They are not really good for fishkeeping - even the website suggests that you change the water just every four weeks!