I'd recommend soil substrate Martin. My nano 7g below is a walstad type one.
One thing I disagree with some "low tech" keepers is the amount of light. In very low light as in 0.5WG of T5 I can't grow anything, just anubias and java fern, crypts are ok too but they grow so slowly that it rather seem they are staying alive instead of growing. Stems won't grow and eventually die. Only floating plants near the surface(including if I float the stems) will grow. I've always had better success with medium light in low techs and no major algae issues at all if any. Lower the light, plants stop growing and then start looking like blanched spinach eventually if they don't tolerate low light. Even my moss won't take off in low light. You can see in Edvert's black cube the consequences of low light on moss, it grows "leggy" and brownish instead of dense and bright green (no offence Edvert the tank looks great otherwise).
Mine is on 11W light 6hrs a day plus direct window light(though it isn't blazing sunshine here but it does every so often and I have to block it for the purpose of taking a pic or video because of the reflection) No ferts bar once potassium so far, it's been setup nearly a year. Dirty glass as it hasn't been cleaned at all for a year too. All I've done is trim the plants. It has a 450l/h internal filter and gets no water changes. Healthy cherry shrimp, berried females, snails and 2 ottos that were added just over 3 weeks ago, daily feedings of fish food for the shrimp and snails and occasional veggies.
I am not saying mine is a better low tech as I've 0 scaping skills but as far as plants go they grow very well.
