Great stuff is an expanding foam crack and gap filler. you can get black or yellow. black is for ponds so its safe but i use yellow also. then cover it in silicone and mash coco coir into the wet silicone wherever you want to fill gaps. wait for it to dry a few days and fill it with a hose outside to get the loose stuff out. the coir will tint the water a little brown for about a week but its all