The best thing you can do is go to a garden centre or aquatic stockist and buy the smallest filter available. One for a small fish tank will be fine.
I tend to leave the filter running during the day. All you really need is a trickle to stop the water getting too turbid. Too much flow may disturb your toads.
The filter will help clean the water but the toads produce quite a lot of waste so water changes are still required - at least a third of the water every week . Replace it with rainwater from a water butt so you won't have to worry about treating it .
You can buy good cheap Aquarium gravel pumps from Ebay. All you do is squeeze the pump a few times and the water is sucked up , no need to keep lifting the tank.
Some pond weed would also be a good idea as it gives the toads a place to hide and also helps to filter the water. I would recommend Java Moss - Ebay is a good source.
Hope this helps !
Enjoy your toads.

Barry