I'm in MN US and am wondering when it would be safe to start my 30 and 150 gallon guppy ponds? They will be placed in partial shade or full sun and I an able to run a 15 gallon heater just to slightly raise it at night next to the heater. A friend's son's friend has guppies and is wanting to downgrade a few of the offspring (free to me) so I will be getting 10-15 plus I have over 20 current gups. What minimum temp can it go down to? It's an average of 40 and 50 at night and 55 and 62 at day. Any recommendations for keeping my ducks and other critters out of the ponds?