Take it outside and spray it out with a water hose, then fill it up with water and put a heavy dose of bleach and liquid soap. Let is set for a day and then scrub the crap out of it.. Poor out the junk spray it out again and repeat with the water and bleach.. Let it sit again and the next day pour it out rinse it really really good until you cannot smell the bleach any longer. Then let it sit another day under the sun then your done... Chemical, manual and UV cleaning..