I've got a bottle and holder set that I bought off of Amazon that I used in Victoria's tank when she first came home that I thought might be worth sharing (I apologise if it's been shared before):
https://www.amazon.co.uk/gp/product/...?ie=UTF8&psc=1
It's fairly light and a small Chinesey managed to knock it over when she got a bit too carried away with digging but it's worth considering as a possibility for a robo.