For anyone struggling to vacuum the bottom of the tank without sucking tadpoles up, I've found a solution. It takes a bit longer, but it's worth it.
I got a long a piece of standard-width airline tubing (the stuff you use for air pumps/airstones, etc) and created a vacuum with it. To do this, I took the pipe to the bathroom and held one end under the running tap. I waited until there was a steady flow of water coming out of the other end, then I put my thumb over the end with the water coming out of it and kept water running into the opposite end until all the air bubbles had gone from inside the pipe. I then blocked this end with my thumb too and brought the pipe back to the tank, still blocking both ends with my thumbs. I held one end of the pipe in the bucket below the tadpole tank and the other end in the tank, and released both of my thumbs to create a steady flow of water coming from the tank.
I managed to clean the bottom of the tank thoroughly and avoid hurting any tadpoles! Because the water comes out much slower, even if the tadpoles swum close to the tube, they didn't get sucked up. The larger tadpoles probably wouldn't even fit in the tube, and even if the smaller ones did get sucked up they'd have a much smoother ride than being forced through the grids and bumpy plastic of the gravel vacuum at high speeds. After cleaning the bottom, I just used a cup to get out the rest of the water that I wanted to remove.