Try better tube insulation and also there a big gap in the grill. Try covering it. A bigger copper heating coil should be also in consideration, or make two heating coils. Or ditch out the grill and make a large coil and put wood inside. No pump required for smaller pools.. Probably You would end up whit multiple insulated copper coils. Stacked car radiators should also do the trick like @mholiv said, but You should put the pump on higher speed so they wont start to boil inside. Be sure that the rubber hoses are high temperature proof. Or just put a thick steel barrel into the pool and put wood or coal inside, just make sure to add insulation on bottom so Your pool wont have a burned hole on the bottom.