S/H and cold winters

[Fredmax]

All s/h plants came inside to 18c lows, the issue was the highs stay at 18c during the day and no higher than 20-21c so the reservoir didn't move (like the 30's which all the s/h plants thrived in). The reservoir sat there for weeks and the algae turned the roots green/black, spreading to or dehydrating the plants.

For the experienced and successful growers what is the temperature range are you growing at?