Also consder that the rock and sand take up volume thereby displacing some water. The rock density/weight is only slightly greater than the water that it is displacing. So for example, if your 60lbs of rock and 25lbs of sand (85lbs) displaces, let's say, 8 gallons of water (just an educated guess) that would mean you are replacing 8 gallons * 8lbs/gallon or 64lbs with 85 lbs so the additional wheight the tank has to support is only about 21 lbs when you add 85 lbs of sand and rock.
As others have mentioned - I think you'll be fine.
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein (1879-1955)