We use gravel backfill as opposed to sand.  Here is a great excerpt from an article written by Jason Hughes at River Pools and Spas, out of Virginia, explaining why gravel is a better backfill than sand.

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"If you talk to enough people or scroll through enough online forums you'll probably come across someone who has developed a bulge in the wall of their fiberglass pool.  We encountered this problem years ago with several of our pools.  This caused us to scrutinize our installation process and thoroughly inspect the pools where these problems occurred.  What we found flies in the face of the standard and accepted method of installing fiberglass pools across the country.  The problem in our case stemmed from the backfill material that we used.....namely sand.  Tens of thousands of fiberglass pools have been installed with sand backfill without incident, and most of our sand backfilled pools were fine as well.  But when the right (or wrong) combination of circumstances align fiberglass pool walls do bulge.  Question:  What happens to sand when it becomes saturated with water?  It liquefies.  That would be fine if the liquefied sand were resting against something other than fiberglass which is renowned for its flexibility.   Fiberglass pools are structurally engineered to remain full of water.  The outward pressure from the water works in conjunction with the structure of the pool to stabilize the vessel.  When sand liquefies it is heavier than the water on the inside of the pool and if the wall is not strong enough to maintain its shape a bulge develops.  High water tables as well as soils that hold water both lead to situations where a significant amount of water surrounds the pool structure and saturates the backfill material.  So does this mean that fiberglass pools are not suitable for installations with high water table or certain soils?  Quite the contrary; I would submit that there is no better pool to have in such circumstances.  You just need to make certain of two things.  First, that you're pool is strong enough to prevent bulges.  And second, that you eliminate the sand backfill and use gravel.  Is there a difference between the strength of fiberglass pools?  Yes, but from the perspective of the consumer it will be virtually impossible to rate the "bulge-ability" of the various pools on the market because all manufacturers claim to be the strongest and most stable.  The one thing you CAN control is the backfill material, and based on our experience, using gravel backfill is the only insurance against wall bulges in any situation with any pool.   Clean crushed blue stone ¾" or smaller is the gravel used in our installations because it has several distinct advantages over sand or even round pea gravel.  First, its properties do not change when it becomes saturated with water so it performs the same wet or dry.  This makes it ideal for any situation and especially in areas with a high water table or difficult soil conditions.  Second, the point on point friction that occurs between the pieces of gravel make it a very stable material on the sides of the pool; much better than saturated sand that liquefies or pea gravel that acts like little ball bearings.    Third, the gravel compacts upon placement unlike sand which needs to be placed in lifts and compacted with water.  This is further addressed in the discussion on plumbing problems below.  You will see that this is a major advantage as well.  Opponents to gravel backfill pitch against it by saying that it's a cheaper material and used in order to cut cost.  In reality it actually costs us significantly more to use gravel.  We feel it is well worth the investment because you can't put a price tag on peace of mind........... Why would the plumbing settle?  Good question!  The plumbing doesn't settle, the backfill material around the fiberglass pool settles and takes the plumbing with it.  Yes we're back to our sand vs. gravel discussion again.  This is great because this is a topic that needs attention.  The problem is that it's very difficult to fully compact sand around a fiberglass pool during installation.  Many times, despite a fiberglass pool installer's best efforts to bring the sand up in 6"-12" lifts and fully saturate the sand with water, the sand still settles some over time.  The plumbing is encased in this sand and as the sand settles, it exerts downward pressure on the plumbing of the swimming pool.  Many installers tie their plumbing up to help hold it in place, but not withstanding, that's still a lot of pressure.  Settled plumbing manifests itself in one way....leaks.  "Leak" is a dirty word!  We want to avoid saying that word, and we do so by using gravel backfill that just doesn't settle!  If the backfill doesn't move, the plumbing doesn't move.  It's as simple as that!"