You can gravel vac sand; depending.
First, you want sand that is fairly large grained... well you want that anyway for a number of reasons, but we will focus on vaccing here.
US sieve number (mm)
Coarse #12 - #30 (1.7-0.6 mm)
Medium #20 - #50 (0.8-0.3 mm)
Fine #30 - #70 (0.6-0.2 mm)
I pulled that chart off a cement website, but it works out the same for all sands. You want grit at least ~0.5mm in size; no smaller than #40. When people tell you horror stories about sand, they had grit smaller than #40.
The other half of the equation is suction. I am assuming you have the standard 2" plastic cylinder with 1/2" hose? Gravity fed from a height of 4', you have enough suction to pull water and small particulates... and general scum, but not enough to lift the sand grains up to the hose. The sand will flow around in the 2" cylinder about 6", but none gets sucked out. You can really jam the cylinder down in the sand and get a good cleaning with little trouble.
I have never had a tank that large, so I really cannot speak well on the filtration equipment. Just try to keep the intakes at least 6" above the sand floor.