Hi, Jazzzz,
if I can add to the excellent comments of three-fingers above:
you certainly aren't the only one to consider doing small Walstads with a couple of small, hardy fish, as it seems many have done so over the years.
However, the Walstad method is fairly specific, and based both on science and on actual experience.
Many people have termed tanks obviously not of this type as Walstads - and some have attributed problems created by ignoring basics to the fault of the method, rather than their own deviations.
I'd just like to quickly note that while surface disturbance will drive off excess co2 from a tank to which this is added, in a natural tank without artificial co2 addition, air disturbance may draw more co2 in from the air to balance natural levels, wherever below this.
I know nothing of this myself, of course, and am too lazy right now to dig for examples, but I've come across a number of different people's identical explanations of this.
The natural tank is entirely different from the high-tech in functioning, chemistry, and aim.
The soil-based, gravel-capped Walstad method involves using a number of different plants, generally as many as is feasible, especially fast-growing stem plants, emergents and floating.
While the photo period is (or was, last time I checked the site) regarded as being of more importance than continuous light during the recommended period of about 14 hours for emergent plant growth, 6 hours of light would appear to fall far short of that generally accepted as supporting optimum plant growth.
Snails are also required, as mentioned above, in achieving a balance - and in cleaning plant leaves.
A number of people have successfully balanced small tanks without water movement, but it's trickier, as plants are then deprived of nutrients otherwise flowing past their leaves, and as tiny speck of debris can settle out and remain on leaves, resulting in injury and algae growth.
But different types of plants are better suited to this still-water environment than others - and marsh plants such as Pennywort and Cardamine lyrata are examples.
If you have your heart set on a grassy plain, have you considered compromising with some Pennywort (duckweed being SUCH a pain) which could be planted to provide length so that while graceful, trailing 'lilypads' could grow across the surface, they could be trimmed back periodically to prevent excessive blocking of light from the other plants?
They aren't an emergent, of course, but they do suck ammonia as does Cardamine lyrata, which can be planted in scattered background clumps without spreading madly through your 'lawn'.
People have done Walstads of single plant types, but it's strongly recommended you follow closely the standard method for your first few times prior to experimental variations.
The water does not necessarily become stagnant without artificial movement, but a good plant mass is required as well as the movements of fish.
Many plants do not do well below certain temperatures, and a remarkable number of grass-type plants sold for aquariums seem to require tropical temps - something to bear in mind in an unheated tank, depending on ambient room temps. on a year-round basis.
Hope some of this makes sense and is of help - it's 3 AM here, I keep trying to think and it's just not working...
Edit: re above - I missed a whole word...
