Glass is mostly silicon dioxide, otherwiswe known as quartz. According to wikipedia, quartz has a density of 2.65g/cm3.
Now take the dimensions of the tank (if you only have an inch ruler, multiply each by 2.54. That gives you the answer in cm. multiply the length times the height, times 2, that's two of the walls. Now the width times the height times 2, that's the other 2 walls. Now the length times the width, that's the floor. Add all 5 up. Now multiply that number by the thickness of the glass panels. That's your volume of glass in cm3
Now assuming that the density of the glass and the density of quartz are the same (they are, roughly), all you need to do is multiply the volume of glass times the weight of glass per unit of volume (that's called density) and you get the weight of the glass in grams.
If you need to give your answer in pounds, divide your answer by 454, since there are 454 grams per pound.
Hope this helps