Bottom line (yes, I'm putting this at the top): The chicks will show you if they're too hot, too cold, or just right. If they're too hot, they'll get as far from the heat as they can and they're loud and noisy. If they're too cold, they'll pile up and huddle close to the heat. When they're happy and just right, they'll be scattered around the brooder quietly cheeping, or sleeping.
I have a standard brooder plate with no settings; I also felt it was too hot for the chicks, so I taped a temperature sensor/regulator to it to see..... surface temp at factory settings was 125-deg-F. I read that's what it's supposed to be, to keep the air below the surface at the right temp. Most people just plug their brooder plates in and never check, and it's FINE. However, I wasn't comfortable with that, so I set the regulator to hold it at 95-deg and that worked fine, too. It still felt HOT to the touch, like burned my finger within a few seconds. The chicks, however, love it. SMH.
I have a brooder hut as well, for chicks 2-5 weeks old, until they get their feathers in. I've used this hut in winter with a standard room space heater (flat, oil-filled radiator type, for small rooms) on a low to med-low setting. I've set it up so that the chicks have a low (3-inch) 'platform' of sorts on both sides, but it's not touching the floor, and placed it near a corner of the hut so the heat would radiate off the walls in that corner. When it's very cold out, they'll huddle up and rotate themselves near the surface, but not touching it. I added a standard heat lamp for the coldest nights (below 40-deg-F), more to make me feel better even if the chicks didn't need it. Everyone grew up just FINE.
Have you ever learned about the reproductive cycle of Emperor penguins? The males incubate the eggs during the coldest part of winter, by resting the egg on their feet and in an abdominal fold of skin. (The females have left to hunt for food.) Incubation is 65-75 days, and the males will go up to 4 months with no food. During bitter winter storms, the males form a huge rotating blob of bodies, packed close together to conserve and share body heat. Those on the outside edges push their way inside the blob, and others are pushed out to the edges. It's fascinating to watch! As the chicks grow and the parents share feeding duties, they often leave the chicks alone - even when they still have just downy feathers. The chicks huddle together in the same way, and stay warm.
I see my chicks doing the same thing when they are cold, and they're fine.
If I were you, I'd find something bigger to use for their brooder, and place the brooder plate like it's supposed to be. They'll outgrow the aquarium soon, anyway.
