Our 28L tank had a school of 5 cardinal tetras, and they werent very active, mainly staying on the bottom, so we added some harlequin rasboras. Wow! they are an active fish, and fill the upper half of the water column. So think about getting different fish that will occupy different parts of the water column. In my opinion guppies are always a great addition. They look great and are easy to keep. (usually cheap too) Oh, and I would always get some cories. They look so cute searching the bottom for food, and help keep the tank clean. Probably panda cories are a good size for your tank, or habrosus. I think the peppered cories get a little larger, but you could still tock a couple.
