30L is a small space - too small for goldfish (which really need large tanks if they're fancy goldfish or ponds if they're commons) and I wouldn't want to put any temperate zebra danios in there as they're highly active.
Personally, I would put a heater in there and have it as a tropical tank - it would be a great size for a male betta with some shrimp or a few male endlers. There are also other options such as microrasbora or ember tetras which might be appropriate for this tank size.
I'm not sure if you're aware of something called fishless cycling. If you're not, it is a cycle which builds up the nescescary bacteria to proccess waste created by the livestock in the tank. Fishless cycling means that the cycle is done before adding any fish or livestock to the tank and is done by adding an ammonia source(household ammonia is popular) and testing the water over a period of time until the bacteria needed has grown. I believe there's a good guide to fishless cycling in the beginners section
to TFF 
