Hi,
I've got a 90L Walstad/Lowtech/El Natural up and running and a 160L that I'm about to do a test with soil only substrate.

The 90L is having some iron problems. I've been trying to up the input of iron via foods since I'm trying to keep the balance without dosing ande there is some improvement. The tank is about 4 months and I used potting soil for cacti. The soil might be the problem since I soaked it and it was already low in nutrients. On the upside there were hardly any algae at all during startup.

The 160L is basically a test to see how the walstad method does without the special redox situation that comes from the soil/gravel combination. I'm going to use soil, 5-7.3cm=2-3" and some clay in the bottom, 1cm=2/5". I'll cover the bottom with leafes. I'm going to add live tubifex to the substrate, keep some detrivore bugs and have malaysian trumpets to airate and mix the top layer. Hopefully the critters will move the nutrients-to-be-recycled into the substrate for better chemical processes.

All in all, I think low-tech is a great variation. It's a not so time intensive and generally slower to react to input tank. Good when there isn't time to keep tabs on a high-tech tank where one can totally devastate the tank if one misses a single day.

If anyone is thinkting of staring a lowtech the only advise, apart from the many availavle, is to initally stock the tank with inhabitants that can hadle O2 going very low. That would be snails like Pomacera Bridgeesii(Applesnail) and labyrith fish, Bettas, Guramis et.al. That way there is more room to experiment with low surface agitaion so you can keep CO2 levels higher. Remeber that the substrate generates CO2 in a lowtech.

Ah well, my 2 cents worth...
cheers
Mattias