A temperature of 74°F is perfectly acceptable for your nursery tank as long as you avoid temperature fluctuations. Due to the small volume of water in nurseries, they tend to remain at the ambient air temperature of the room the tank is in, which means there will be temperature fluctuations (warmer during the daytime and cooler at night) unless a heater is used.
Many nursery tank designs, such as divided nurseries, palpins and other in-tank nurseries, and many kriesels or pseudokreisels allow the heater to be kept in the host tank, separate from the inner rearing chambers, so the heater never comes in contact with the fry. If that’s not possible with the basic nursery tank you are using, Barbara, there should be no problem placing a small heater directly in the nursery tank itself. The barrel of the heater is far too large in diameter for the fry to hitch onto, and even if they did, modern aquarium heaters are designed, insulated and engineered so that there’s really no danger of a heater burn occurring. To eliminate even the slightest possibility of such an accident ever occurring, various sleeves and heater guards are now widely available as well. So don’t hesitate to incorporate a suitably small heater into your nursery tank in order to stabilize the water temperature.
Best of luck with your first batch of fry, Barbara! It’s encouraging that some of the newborns are hitching already. Here’s hoping your brine shrimp hatcheries are so running in high gear and producing all the Artemia nauplii you need.