Doing this for a living fulltime, I suggest a single Siamese fighting fish. You don't require filtration or heaters (unless really cold in the winter months) for them as you would for other fish commonly available in the pet trade.
Though keeping a single fish may seem boring for the keeper, you can nicely decorate it to make it visually interesting. Also, one can use OASIS (green foam block from gardening centres), cut a 1-2" wide strip and secure it to the back trim of the aquarium. There, you can put houseplant cuttings to add an extra dimension of "natural" growth. It will also act as a filtration system
. Spider plant runners on the water surface with the roots dangling down will make somewhat a natural environment for them as they will blow bubble nests.
They come in different color and finnage morphs, ie. crowntails, double tails, combtails, etc.
Pics
They aren't very dirty per se and water changes (25-50% via
gravel siphon) very 2-4weeks is plenty enough in that size set-up.
We'll...that's a start and we'll go from there
I don't suggest reptiles as they do require other specialized products such as basking lamps to raise temp for digestion and UVA/UVB emitting lamps for vit D3 synthesis.
JM2C/E