Now for my 10 gallon tank, I'm able to keep three gobies and some invertebrates, but I must go very slow or risk serious damage to my tank. This is why nanos are still not recommended for the beginner hobbyist. Current data shows that it will take me at least 6 months to finish this tank. Why so long? I shall explain in the next paragraph about the "bio-load." That is all technicalities with fish in both fresh and saltwater, but if you have no interest or already understand it, then feel free to skip it.
The bio-load is another term for capacity of the tank. This is calculated by a variety of factors included, filtration, oxygenation, plants, organisms, etc. The most important factor in the bio-load is bacteria. Specifically Nitrifying Bacteria. Fish produce ammonia (NH3) as their waste. This is highly toxic to fish. It's not a problem in the wild due to currents and the amount of gas exchange happening daily around the world. There is simply enough water to balance out the ammonia and the fish. Now in an aquarium, we have a closed system. We control everything that happens with that tank. So the amount of Nitrifying Bacteria must be greater. What is this bacteria? It is the organism that breaks down ammonia into nitrite and then oxidizes it into nitrate which can be removed through gas exchange and water changes. This is why open top aquariums are less maintenance, there is more gas exchange. Nitrates will build up over time and in large quantities can kill your fish, but there is almost always small amounts in every tank. Oxygen levels also play a role on the amount of fish you can keep since fish need to breathe too. Plants have the effect of using up oxygen and producing carbon dioxide at night when the lights are off. In saltwater tanks this can be managed by having the lights in the sump alternate with the tank lights.
This is the rough outline of why it's taking so much time to get it set up.