Many people who have large reef aquariums will have 100s of lbs of rock and substrate in there aquarium without any problems
I have a 180 gallon aquarium with about 200 lbs of rock in it and about 60 to 80 lbs of substrate.
You can also put some egg-crate on the bottom of the tank (before adding the substrate and rocks) to help spread out the weight of the rocks over a more even area.
If you take your time to do the research FIRST, you can successfully set-up and keep ANY type of aquarium with ease.
"Not using a quarantine tank is like playing Russian roulette. Nobody wins the game, some people just get to play longer than others." - Anthony Calfo Fishless CycleCycling with FishMarine Aquarium Info [URL="http://saltwater.aquaticcommunity.com/"]
Total weight, while not irrelevant, isn't your prime concern; rather, weight per GIVEN square foot is what you first handle. Having a very high pile of rocks all in one small area of the tank could cause major issues. However, the same total weight spread over all the tank would have almost zero impact.
Also, rocks displace water so their weight (not mass!) is reduced by the volume of water they displace; so a rock that has a cubic gallon of volume and has a dry weight of 18.8 lbs would weigh just 10 lbs in the tank. This does count for total weight in the tank. Bottom line: the tank can hold rocks very well just don't allow a rock to sit directly on the glass!!! That can cause two problems - it can scratch the glass which can cause failure and can concentrate all the rock's weight in two or three tiny points leading to failure.
Knowledge is fun(damental)
A 75 gal with eight Discus, fake plants, and a lot of wood also with sand substrate. Clean up crew is down to just two Sterba's Corys. Filters: continuous new water flow; canister w/UV, in-tank algae scrubber!! Finally, junked the nitrate removal unit from hell.