Well at the moment I just have plain aquarium gravel, that's in a lot of different shapes (all are bigger than Luckys mouth at the moment) can't remember what my dads gravel was like? But I was thinking of getting pea shaped gravel for the larger tank anyway as the gravel I have won't be enough and I quite possibly brought the wrong sort for the 20 litre XD
From what I've read some people think sand is good, they think Goldfish will spit it all back out and that it's more hygenic because poo sits on the top of sand, rather than get buried in some gravel. But as you said casts are more common in sand tanks, so they must be digesting some of it.
Also when it comes to cleaning, I think you can still use a gravel cleaner, but you have to be careful with it and only hover it above the surface of the sand.
Another thing I read is that with sand, you must only use a thin layer (so you won't need to 'dig deep' with a gravel cleaner), otherwise if the sand layer is too thick, there will be 'dead pockets' where oxygen depletes and no bacteria can really live. So you have to 'stir' the sand weekly, or get something to do it for you i.e. trumpet snails.
I also read a rumour (not yet found any proof) that 'methane bubbles' can form in sand that is too thick and explode
The people in the forum were saying that I could use sand as a substrate, but I don't see what nutrition sand could give to a plant? The sand just seems like a lot of hassle if you ask me XD