How old will they be? Just hatched or a few weeks old? That tank is probably about 36" x 15", somewhere in that range. How many chicks are you getting, 4 or 6. I forgot? That is plenty big enough either way. Where are you putting it, somewhere it is climate controlled or somewhere you can have wild temperature swings? I raised mine outside where it sometimes went from below freezing to in the 70's in a day and a half. It can be done but in a climate controlled area is easier.
Too much heat can be as dangerous as too little if not more so. My ideal brooder has one spot that is warm enough in the coolest conditions and cool enough in the warmest conditions. I find that straight out of the incubator chicks are really good at self-regulating their heat if you give them the option. Some mornings my 3' x 6' brooder might have ice on one end but the chicks would be on the toasty end. When it was warmer they might be halfway down the brooder away from the heat where it was cooler.
How warm is warm enough and how cool is cool enough? There are formulas out there to calculate heat, a typical one might start with 90, 95, or even 100 degrees the first week and drop it 5 degrees each week after. That's a very safe guideline for your warmest spot that will keep you out of trouble but they can typically handle cooler. I find the first two or three days they spend most of their time under a broody hen or in a warm spot. After that they go exploring a lot, even if it is pretty cool. They go back to that warm spot when they need to warm up. How cool is cool enough? Broody hens can raise chicks in really warm weather or even if there is snow on the ground. You have a big window that can work. If you are in your house air temperature would be about right. The main thing is that they can get away from the heat if it gets too warm.
Your chicks will tell you if it is too hot or too cold. I trust them more than any thermometer. If they are as far from that heat source as they can get and maybe panting, they are too hot. If they are crowded under the heat and giving a really pitiful plaintive cheep they are too cold. If they are kind of spread out you nailed it. At night they tend to sleep in a group in a warm spot, that's more for the comfort of sleeping with their buddies than because they are cold. Don't let that fool you into thinking they are too cold.
There are different ways you can provide that warm spot. I'll assume you are inside. The bulb you got with your heat lamp is probably a 250 watt, that's probably too warm. You might only need a 75 or 125 watt. You don't want to turn that entire aquarium into an oven. Other than the wattage of the bulb a way to control heat in the brooder is height above the chicks. That may be a little challenging inside those glass walls. If it gets too hot in the entire area you can maybe hang it over the end so some of the heat goes in and some goes outside the brooder.
If you use a heat lamp throw away the clamp that came with it so you are not tempted to use it. Use wire (not fiber or plastic string that can burn or melt) and firmly attach it so you, your dogs, or that ghost that's decorating your wall cannot possibly knock it over. Then use a second wire and do it again. I think that clamp is responsible for a significant number of the fires you can get with a heat lamp. I used heat lamps in my outside brooder and had no more worries about it starting a fire than anything i would run electricity to but I was very careful.
There are several other ways to provide that warm spot. They come with their own risks and benefits. Heating pad caves are real popular over on the chicken forum. If I were raising a handful of chicks inside my house I'd probably use a heat plate, Brinsea sells a popular model. I'd look on Amazon instead of Craigslist. It's basically a raised heating plate that they can get under. I've never used one but they seem so simple.
I'll write some more chapters on your other questions soon. I know I can make it sound complicated but it really isn't.
