One great way to kill everything in your tank except your plants is to either get a tank of CO2 or some dry ice and place in the tank and "gas" your tank. All the critters will die and the plants will actually thank you. Make sure it is seal 100% and gas the heck out of it so the CO2 gets all the way through the substrate. Once it is done, unseal the tank and let it air out really well and replace your gecko(s). That way there is zero residue of any kind that can harm your gecko.