Ideal water temp
We are in the dead of winter and my grow room gets a lot of ambient heat but it is not hot. It doesn't make much difference (until now) because all the seedlings are in a chamber where it stays warmer. But I want to start some toms in a hydro unit and the water now is 20C, 68F. The air temp is 72F so the water will warm up a bit in time.
These are small seedlings that really don't have any true leaves yet, just good roots in rockwool. I figure plants get set outside in May when the soil temp is ~65 and the nighttime temps can drop well below 72 so don't see this as a problem.