How to increase substrate depth??

[Hooky]

As per title, I want to increase my substrate depth but not sure who best to do this without disturbing the tank too much. I currently have a substrate depth from 2.5cm to 5cm at the back but am now looking for a 4cm to 8cm depth to try and help the heavy root structure of my cryptocoryne wendtii brown / green, which seems to have some stunt growth. I am using dennerle scapers soil with a grain size of 1-4mm, but would like something cheap to cap this with purely to increase depth.


Any help would be much appreciated

 

[splatteredbrainz]

Pool filter sand to cap it off

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