Hi Mike,
Yes, the tank is essentially a Walstad type setup with added riparian plants. The substrate was very cheap, about £10 in total for the soil and sand (compared to at least £100 for the same coverage of one of the branded aquarium plant soils). I actually use ADA Amazonia in my smaller tanks where the cost is not prohibitive… it has the advantage of good aesthetics, being easy to work and having a good particle size to prevent anaerobic spots. However, normal soil is just as good for growing plants really.
Many soils will lower pH, however you need to check this before planting… for example I have recently learned that John Innes number 3 contains added lime, which can raise pH and hardness dramatically. So worth testing some before hand – I have heard that Miracle Grow aquatic compost is good (I think Walstad uses this often). But not too expensive to buy a couple of different types to test, if you have a garden then you can always use unsuitable ones somewhere else!
As always, unless you mineralise it first you should also expect ammonia to leach from any soil for up to a month after it is submerged.