Silica sand might be a little rough depending on how sensitive the fish are in practice. If you already have the sand, I say go for it and just keep a look out for abrasions on the fish. If there are abrasions/irritation/etc then I would switch to something a little softer, but until then I'd say go ahead. If you don't already have the sand, I would go straight playground sand. It's fine enough to be soft.