For small tanks internal filters are more efficient for heat conservation (in cooler months), i found polystyrene insulation definitely helps, unfortunately open top tanks will struggle to keep heat in, (better in recent uk summers at keeping temperature down though), my new setup has been designed with heat conservation in mind (wier fed external filters, no external pipes, 20mm polystyrene covering rear of tank and inside of pelmet/lid), will probably have to use chiller during the summer, but hopefully not too often