Can I use lead to weight down corner filter?

[MrNitrite]

I got Lee's corner filter for the 2.5 gal betta tank. If I use gravel to keep it from floating, I have to put so much in there there's not much room for media. What about using lead? Is it safe in the aquarium? If not, what would you recommend? I even tried a couple of small rocks, but it's still too light.