I have come back to fishkeeping after several years. I originally only kept cold water fish, but due to inexperience and ill advised enthusiasm all my fish died. I have now decided to resurrect my hobby by keeping aquarium fish - still inexperienced, but with more circumspection. I have fluval flex 57 ltr (12.5 gals) tank. I currently have just a couple of mollies and 5 serpae tetras. I was wondering what if anything else I could add to this tank. I also have a biorb 30 ltr (I know!) which I have just set up again. What would you recommend would be the best fish to keep in it. I have added a compatible heater.