The convection current from the water should be quite effective. I'd love to try out solar heating in the greenhouse - I was thinking of having an old radiator painted black outside the greenhouse and some hosepipe underneath the floor zigzagging through gravel or something. Idea is to warm the gravel during the day and heat the greenhouse at night. I need a permanent place for the greenhouse first (and a better quality greenhouse to make it worth the effort).

As soon as I get my hands on a freecycled radiator I'll give it a go. I have a small solar cell pond fountain that might do for a pump if convection isn't enough - the pond fountain came to grief when the mr mowed the cable.

__________________
You are a child of the universe,
no less than the trees and the stars;
you have a right to be here.
Max Ehrmann, Desiderata

blog: http://allyheebiejeebie.blogspot.com/ and my (basic!) page: http://www.allythegardener.co.uk/