Unless you are dealing with big ranid tadpoles, tadpoles in general are very sensitive. Depending on how many you have left, I would recommend gentle water filtration.

1) Get a really big tupperware container (the size of a 40 gallon tank, these are available at walmart, target, wherever). Cut a hole somewhere near the top of this container, and silicone in pvc piping that leads down to a bucket, make sure the bucket is clean of chemical residue etc. The piping should reach the bottom of said bucket. Fill the bucket with sand and gravel about a third of the way, carbon and zeolite (ammo chips) the other thirds, and cover that with filter floss. Cut a hole in this bucket, near the top above the filter floss, and use PVC Piping, siliconed in place to transport water to a second bucket.

What this does is brings water down using gravity to a filter, the water percolates up through the filter media as the bucket fills. The inflow will also always equal the outflow, though you may need to adjust the pump rate to make sure it does not run dry.

2) In the second bucker, put aquatic plants (for additional biological filtration) and an aquarium pump that pushes water back up to your tank using aquarium tubing, and pvc pipe at the end. use PVC pipe at the tank end, so you can cap and and drill holes in it to gently spray the water along the side of the tank, so the inflow does not injure or stress the tadpoles. This also oxygenates the water without hyper-saturating it--which can kill the tadpoles.

3) You have two options for keeping your tadpoles out of their filter

You can cover the outflow pipe with window screen, and keep the tadpoles communally, or you can insert sections of wide diameter pvc into the tank, and house them in batches inside the main tank.

This is how our Xenopus lab raises their Xenopus (though an entire room full of these setups), so I know it works.