Vinegar works for hard water stains. Be sure to rinse well afterward. A razor blade scraper will remove built up crud in general faster than most methods, be careful not to damage the silicone though. Some scrub pads can be used, but be careful, you can end up scratching the glass.
You may find that where the water level was, there's a whitish discolouration that does not want to come off. It's probably etched, and if so, the only way to get rid of it is to polish it, and that is far too much effort to go to. It won't show when the tank is filled, and minor scratches on the inside of the glass usually don't show either once it filled.
There's more than one kind of sand. If you mean cheap ordinary sand, pool filter sand works pretty well. Builders or concrete sand is fairly sharp, which can be hard on some fish, such as catfish or loaches that have barbels. And that kind of sand can also be pretty high in lime, which is to say, calcium. That's not necessarily a bad thing, but if you are wanting a soft water or acidic tank, sand full of calcium will leach the calcium and change the water parameters.
You can check for calcium.. just pour some vinegar onto the proposed sand. If it bubbles up, there's a lot of calcium in it.
Some have used play sand, some have used the sands made specifically for aquaria. They are more expensive but are not going to alter water parameters at all. Some are made specifically for planted tanks too, so plants are going to like them better if you plan to plant the tank. Flourite is one, comes in a sand type and a coarser type too.
If you're planning on salt water, there's silica sand, which is very white, very fine, almost like sugar, but it is not so good for fresh water though. Most ordinary sand would work for salt water too, because the leaching if it had lime in it wouldn't be a problem in salt water.
