The thing you need to bare in mind is that 620T tanks are quite squat for their volume, they are basically two typical 2-foot long tanks stacked on top of each other. Therefore, tankmates should be chosen with care i.e. nothing that likes a lot of space to use fast turns of speed.
In mine I currently have...
2 African Butterfly Fish (with floating Canadian Pond Weed to create boundaries and security)
4 Upside Down Catfish (managed to get a new one today, still aiming for a 6+ group ASAP, as they are social like cory catfish)
1 Hypoptopoma sp.2 catfish
1 Phyllonemus or Lophiobagrus catfish from Lake Tanganyika
9 Golden Eel Loaches (which I barely see anymore, unless I go through to the lounge at silly times in the middle of the night)
~12+ Rabbit Snails (various species, breeding like rabbits!)