A local LFS chap suggested me to use this product. It is used in ponds, etc and its fish safe -
https://www.amazon.co.uk/gp/product...fl_title_73?ie=UTF8&psc=1&smid=A3P9O9FZTPIIYP for the back wall. and to make it look much more natural, couple of ideas -
1) once its almost half way through drying up insert different shapes of rocks/stone and you can install frags later on them
2) once its dried up, apply a thin layer of silicone (same used for building fish tank) and then dip the whole thing into the sand, and any excess it will come off once its all dried up. apply frags as you want.
my 2 cents
