Thanks for the feedback the glass catfish is the only fish I'm set on getting the other fish I'm still considering. So with the sand what type a of sand fine grain and what sort of colour.
This species is a shy and rather nervous fish, quite sensitive to water parameters and highly susceptible to changes in water parameters or conditions. This very unique and interesting catfish must be kept in a group (your intended 12 is perfect), otherwise it may refuse to eat. It also cannot have boisterous tankmates for the same reason. A thickly-planted aquarium providing some swimming space with a gentle current is essential to calm this very timid fish. Floating plants will significantly assist in keeping this fish less stressed.
You can use specific aquarium river sand, which gives you a choice of colours, just avoid at all cost white as this without question will affect this shy fish (substrates are dark which allows fish to feel less obvious and thus less stressed). A neutral dark tone, like buff or dark brown or dark grey is best. Or for less expense but certainly no issues, quality play sand--in the UK Argos is highly recommended by other members. A "normal" buff tone is fine, you can add dried leaves, and chunks of dark wood to break up the sand and these are both natural to the fish's habitat. Collect dried leaves in the autumn (excellent time of year for us northern hemisphere aquarists), from the ground so they are dead dry, under trees like oak, maple, alder, birch, beech are safe.