Deep threat
Beware the alleged hydrogen sulfide pockets trapped deep in the substrate. Have no idea if they're real or not but given the title of the post had to offer something.
Depth depends on what you're doing, but rarely cause for deep substrate unless you have burrowers, botanical demands or extreme design criteria. I've moved away from deep substrate (1' -2"+) to progressively shallower layerings so that anymore it's often only enough to cover the glass to a depth of 1/2" or less.
As most of the plants I use (anubias, moss, water sprite, wisteria, hornwort, java fern) don't require much if any "soil", it's not a problem but also have swords, cabomba, hygro, cryps and a few red-leaved species growing well in an inch of sand.
Substrate should first and foremost suit the inhabitants. Secondary concerns are landscaping and maintenance. Less substrate means more water volume and less cleaning. Have found that one can create berms of stones and wood to contain areas of deeper substrate so as to cover potted plants/provide more areas for rooted plants - also varies the topography without having to utilize huge amounts of substrate.
First and last time I went to buy pool sand (Danny's Pool in Crofton, MD) it was crushed limestone - not good. Instead I stopped at Home Depot and bought 4 slightly perforated bags of play sand at half-price. It's finer than regular sand and takes some work to rinse clean so that all the finest silt gets flushed out, but it's a great semi-bright bone-white color (easily modified by addition of other materials) that sets everything else off nicely and Corydoras love plowing around in it.
The other posters are correct - less is more, one bag of anything should be more than all you need to cover a 55 tank's 4 sq. feet or 576 sq. inches. If you figure 1" depth that means 576 cubic inches - 231 cubic inches per gallon, and Voila! - almost exactly 2.5 gallons of material.
