ACRYLIC or GLASS ?

Acrylic
Acrylic is apetroleum based polymer that is poured and molded into sheets in its liquid form. It can also be referred to as plastic, but it really has a different quality than the type used to make toys.
There are different types of acrylics. One type is known as "cell cast" which is the oldest method known for making an acrylic sheet. This method consists of taking the liquid material and pouring it between two pieces of very highly machined and polished glass that is referred to as a mold, cast or cell.
There are two different methods used for cell casting, vertical and horizontal. That is, the acrylic is either being poured vertically into the cell or is sandwiched between the glass.
The other peocess used to produce acrylic sheet is continuous casting. In this case, the material is pressed out by giant stainless steel roller machines.
For aquarium uses ( tanks, wet/dry filters ) only cell cast sheets should be used.Continuous casting causes acrylic sheet to have waves in it.
The most commonly used type of acrylic sheet is plexiglas G, which offers the best overall balance of properties. These properties, not all of which necessarily apply to home aquarium usage, include weather resistance, the ability to withstand sun exposure, light weight (less than 50 percent as heavy as glass), high impact resistance, ease of fabrication, it can be heated and formed( but, stiffens and retains its shape when cooled), dimensional stability ( does not shrink or deteriorate after long periods of use), good U factor ( acts as a thermal insulator), and excellent material also available in translucent and opaque colors).

Glass
Glass is made from sand. More accurately, this transparent material comes from arecipe that requires frusing silicates with soda or potash, lime and sometimes various metallic oxides. The oxides or fusible pigments can give the glass ides or fusiblepigments can give the glass almost any color. Because of its flexible quality when heated, glass can be manipulated into many different shapes, although it is not quite as versatile as acrylic when it comes to aquariums.
Althoungh glass in general can brittle and often has greenish cast to it, there are many reasons why it remains the preffered material for making aquariums. Above all, it is less expensive, it is readily available, it doesn't scratch very easily, and it doesn't melt or burn if there are problems with aquarium heaters or lighting.
There are two kinds of glass used to make aquariums. Glass is bought by manufacturers in sheets, but one type of glass known as sheet glass, refers to glass that has been rolled. In the early days of aquarium manufacturing, most of the glass that was available was rolled sheet glass, in which the silicone was melted and poured and then rolled between rollers to make a piece of glass of the desired thickness. The problem with this method is that the flatness of the sheet is hightly dependent on the flatness of the roller.
The more current process is called float glass. The molten glass is floated onto a layer of mercury and cools perfectly flatt - a uniform sheet with out blemishes.

The value and Related problem Of Glass And Acrylic
Glass aquariums are not without their own advantages. Although it weighs more, glass is much more resistant to scratching. The variety of glass tanks available in terms of size and dimensions is generally much greater than acrylic aquariums of the same volume, and the prrices for these tanks are considerably less.
Acrylic wasn't always a good choice. There used to be two major problems with it. First, overexposure to light would cause the acrylic to change color, from clear to yellow or orange, and once this occured, the acrylic would start to haze. However, acrylic is now produced with atreatment called ultraviolet stabilization. this neutralizes the chemical that reacts with light, so acrylic aquariums stay clear permanently.
The second problem with acrylic in the past was that it scratched easily, and the material was prone to all kinds of little hairline fractures. Again, acrylic manufacturers have changed the formulas of the product so that it is much more scratch resistance these days. Should the acrylic become scratched, there are many products on the market to take scratches out.
Good quality acrylic tanks use material of the appropriate thickness. Acrylic absorbs moisture, which causes it to become heavy increasing the chances of bowing. Based on the height of the aquarium and the lenght, the appropriate thickness is selected that will keep the acrylic as flat as possible.
For the same volume, acrylic tanks are more expensive than glass ones. This difference, however, is much less for aquariums of 150 gallons and up. For very large tanks, acrylic is a better choice, and because acrylic offers up to four times the strength of glass, there is greater margin of safety.
The clarity of acrylic, particularly for large aquariums, is a distinct advantage, and acrylic aquarium are easy to drill to accommodate filtration system requirements.

How to look quality
With either type of tank, you should look for any obvious problems before purchasing an aquarium. Acrylic tanks, for example, should be scratch free and not have any rough edges or bowed panels. When inspecting a glass tank, look for consistent seams of silicone and make sure that there are no large bubbles in them. Look inside the aquarium, you shouldn't see any voids where the silicone is missing.
Good quality glass tanks are constructed using the best quality silicone, which avoids stress cracks or slow leaks while ensuring a good bond between built aquarium will last almost indefinitely.
The choice of glass or acrylic is one that depends on those factors that are most important to you. Acrylic is lighter and clearer, but glass is much more scratch resistant and less expensive.