It is also possible to heat the greenhouse with fire(if u have a way to keep smoke from getting in)
Nice pond. Try spraying your water lilies with BT(bacilus thur.)once a month and that should keep any bugs off them. Be sure to keep them fertilized(monthly) and they should be good to go
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I would also avoid getting more than 1 Koi. Other fish to consider if u go the heated routearadise fish,studfish,lyretail killifish and a fewvpeacock gudgeon and Amano shrimp