[Michael]

I agree with Sumthin Fishy. I made that same mistake with my first
Walstad tank, also by accident. The tank was eventually successful, but it took longer than normal to become fish-safe.

That tank is still going, with great plant growth and healthy fish. But it is the only tank that ever bubbles hydrogen sulfide from the substrate. I manage it with strong, deep rooted plants, and ocasionally poking the substrate with a chopstick. And I'm seriously considering Malaysian trumpet snails.

In all my later tanks, I've used no more than 1.5" of soil, and often less.