There's a project running in the Philippines where they use old plastic bottles inserted in the roofs of shanty town dwellings to give light inside. From what I remember (I saw a video online but can't remember details) they have two tricks too make it work better than just a small glazed hole. Firstly, they fill them with liquid, which diffuses the light and provides a better spread of illumination in what can otherwise be very dark rooms. Secondly, they don't use plain water but mix it with something (might just be a bleach solution) which avoids the algae problem.
On the water topic, I use a black plastic bin of water in my polytunnel. In theory at least, it acts as a limited temperature buffer. If nothing else, it provides a handy source of water (and that turn over of water, plus the opacity of the plastic, avoid algae problems).
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