Lewisias need a compost which is free draining. I use half and half of John Innes No, 3 (that is a soil based potting compost) and 5cm. grit. No sand in there of any description. Watering is done from below by the way.

Are you wanting to grow lewisias from seed or plant out existing plants? In either case I'm not sure why you've been advised to use river sand.

If you are growing from seed it needs surface sowing. A proprietary seed compost with the addition of horticultural grit (fine grit). Fill pot with this mix almost to brim and add some gravel on the top, sprinkle the seed over this and leave to germinate - can be started at any time of year but seems to work best in autumn.

If you are potting up plants or planting in your garden make sure it is a good free draining compost with no peat. Lewisias hate being wet and whilst they love the sun they like to put their tap root down into the cool substrata.

The adviser probably means river rather than beach sand. I use a little coarse sand in my lewisia compost but mostly grit. I think that Owdboggy uses 5mm grit rather than 5cm ;-)

Whoops!
Actually I use ground up ceramic stuff, but it is easier to get 5MM grit.
Acquaintance who use to grow the L. cotyledon commercially by the million, actually grew on on Rock wool. Murder to get them off it into a decent compost, but they survived and sold. Also he used to water them with overhead sprinklers.

Can I butt in here for some advice please?....
I planted those L. cotyledon into a terracotta pot in the autumn....I can't remember what compost I used but did add grit to it and put the grit on the surface. They are coming on fine and have flowers appearing. If they prefer watering from down below - how do I manage this? - I have put crocs at the base of the pot therefore I am wondering how does the water soak up from the base?
I know this sounds like a stupid question, so sorry for sounding stupid. I really didn't expect these to be successful but as they weren't very expensive I thought I would give them a go. I guess the dry weather has helped me out somewhat :))

The reason for watering from below is that then water does not sit on the crown of the plant and cause it to rot. So the answer is simple, don't get water on the plants leaves when you water it. The ones I grow in pots are watered very carefully round the edge of the put, under the leaves (when I can be bothered that is).
Or do as the pro. did and water first thing in the morning so that any water on the leaves evaporates very quickly.

Thank you owdboggy. I'll try my best to be careful :)

many thanks all have the plants are comming from suttons on line will try your ways ,will let you know how i get on again many thanks.