Lakes are in constant water change.
The nitrogen cycle, the sulfur cycle, the carbon cycle (many other cycles we cannot replicate in a closed system) evaporation, precipitation, terrestrial forests, algae, bacteria and other aquatic plants are constantly recycling nutrients.
You might get away with a 50 gallon aquarium, so thick with plants you cannot see thru to the back, and 1 Betta, or only one guppy for a while.