Leak in bottom of Intex ultra frame pool

I am in the process of setting up an Intex Ultra-frame 16' round pool I got from my neighbor, who purchased it new 1 year ago. I patched one small hole on the bottom before I started filling it up. After I got about 5-6" of water in the pool it appeared to be slowly leaking from the bottom so I stopped filling it. There are two seams that run across the bottom of the pool, effectively cutting it into thirds and I used a syringe and some food dye to check these seams for leaks. I also checked the patch I had just applied. So far no luck in finding the leak. How else do you find a leak? THere is still about 5-6" of water in the pool.