I have red clay, I use it in my topsoil which is under the substrate, but apparently it's not enough for cryptos, as I remember Flourish tabs does not have the necessary elements, right? Osmocote plus is available but only for import, it will take a few weeks to arrive and it's a bit expensive. Thank you for your help.The big thing is that the nitrates, or other nutrients, that you measure in the water are not necessarily the same available to the roots.
Fish waste is more than just nitrogen, so what you can see in the substrate or filter may be useless .
Anything you can put into the substrate should help. Even if designed for terrestrial plants,. If covered correctly will not affect the water.
In the US, osmocote+ or Jobes plant spikes would be the simplest budget solution. Or DIY if you have red clay powder and the different plant nutrition salts.