Your problem will be if the critters laid eggs . If you can you need to take the end of the fill pipe off the tank. Once off the tank you will need to rinse it out thoroughly to remove any eggs in the filler tubing, then you can sanitize the tank after replacing the tube. When you sanitize see if you can dump water straight to the ground from the tank without going through the pipes or water pump the first time in order to flush eggs out of the tank. The second time then put some heavily bleached water through the pipes