Hi, Kitty,

You were trying to combine the Walstad "slow growth" method and a fast growth, high light approach. That can't work.

The standard "Walstad" approach calls for low light and only those nutrients that the fish and the soil provide, with infrequent water changes. "Fast growth" is the opposite of that.

Your nutrients zeroed out because the very high light gave the plants a huge appetite. They consumed what food there was, and then starved to death.

As Tom said, both approaches work and each has advantages and disadvantages. But you can't do both of them in the same tank.

The "slow growth" approach can be very rewarding. You might think about giving it another try, with less light and, of course, a soil substrate.

Good luck!

Bill