Pottery clay is crap and can be more of a detriment than a good thing.
Clay derived from one's homestead soil is better, grated and added to a soil mix.
High CEC substrates can be a plus depending on one's situation.
The tighter a cap is over soil the better, less transfer to the water column.
This depends also on the amount of nutrients in the soil.
Don't wish to begin a debate on the soil combinations I've tried up until now either.
Most get at least a 1 year run, some get 2-4 years.
FWIW the number one factor I've seen over the past 2 years is a reduction in KH levels.
Either RODI water introduction or HCl. Experimenting both ways in 2 tanks.
How low? Greater than 0 but less than 1dKH, GH is always under 6dGH.
In a low KH environment an inert substrate is adequate with water column dosing only.
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Growing is not that difficult.