Estimated Filled Weights Of Common Aquarium Sizes

[Chad]

And not all substrates are created equal. By volume they might displace a gallon of water, which weights around 8.25 pounds but if the object has a higher density then it could actually be adding weight even thought it's removing water. 

 

[Jeremy180]

And not all substrates are created equal. By volume they might displace a gallon of water, which weights around 8.25 pounds but if the object has a higher density then it could actually be adding weight even thought it's removing water. 

While the average density of substrate seems to be around 1.5 times that of water, I'll err on the side of caution and use fine sand in my calculations, Which has a gravity of 1.99.
Of course for simplicity, I'll round that up to an even 2.

 

[Chad]

Aside from substrate there are aquascape. Plants, liverock in a reef tank, stones, etc. Not all sand is created equal either. Some sand can be silica based, and other sand can be calcium based which are going to have different masses. The materials that compose sand differ based on location. Silicon dioxide is going to be common but so is sand created from calcium carbonate (coral skeletons).  
 
Here are bunch of different substrate weights: http/www.anchsand.com/default.aspx?tabid=131

 

[OldMan47]

Almost anything you add to a tank will add weight unless it is something that floats. That includes the substrate which is heavier than water. It sinks doesn't it? The submersible heater adds weight, the filter adds weight and hangs above the tank on a small tank but a canister filter sitting on the floor behind the tank adds nothing. The fish swim at all levels in the tank so they are more or less the same as the water that they displace. Don't over-complicate it. If you have 50 pounds of gravel in a 29 gallon tank, just add 50 pounds to your total and you will be plenty close enough with your estimate.