Ya, seeds generally wake up (sprout) faster when it warms up in nature, they think it's spring. 40 degree (Fahrenheit) water is cold. I try to keep my seeds and seedlings about 80-85 degrees. Most seeds wont sprout well if at all at 40 degrees. Once they sprout they still need to be kept warm or they wont grow very fast. I have some seedlings right now I am having that problem with. I need to keep them outside because I don't have any artificial lights that would allow me to keep them inside. So once I get the seeds to sprout, I put them in a small amount of potting soil. Then when they break through the soil I need to keep them outside where they can get light.

Most of the seedlings have not grown at all (noticeably anyway) in over two weeks. Even with keeping them in a covered tray with a heating pad inside. The heating pad helps but the tray has too many plants (seedlings) in it to do a good job at keeping them warm enough. I am not sure about nighttime but the thermometer I keep inside the tray with the seedlings typically read in the 50's or 60's (even while covered) during the day. Last few weeks we have nighttime temps in the mid 30's (even a few below freezing), and daytime temps in the 40's and low 50's (for us that's a long cold snap). Bottom line, if I were able to keep them warm they would be much much bigger by now.

I'm trying to start so many plants so I can sell them in early spring. I save all the seeds I can from all the fruits and veggies I eat, so I get the seeds for free. I live on a main street that gets a lot of traffic, and when the weather starts to warm up (late January and into February) there are people every weekend that go out looking for all the garage sales in town. And that's also when everyone likes to start planting things. So I plan to have some garage sales myself, only I will be selling plants.

Many seeds come with germinating information on the back of the package which for some unknown reason usually prescribes a specific depth for starting a specific variety, but leaves out temperature details which seems to be much more useful information in my opinion. I've got 5 varieties of peppers (4 each) germinating at 82-84 deg. F in a covered tray over a heating pad. Next month I fire up the water bed in the spare bedroom when I start my annual Queen size seed starting ritual. I have several neighbors that are always willing to take the extras.

As far as air temp. Vs. nutrient temp. in a DWC type system I think nutrient temp. is more important. My Hydro systems are under florescent lamps in a room that is not heated and often drops below 45 deg. F., However they are ultrasonic fog/DWC systems. The foggers themselves give off heat bringing the solution up to 75-95 degrees. In conclusion I would have to say yes, add a fish tank heater to your system. I think it would be much more efficient and practical than heating an entire greenhouse.
Good luck