I think your aquarium could work, at least in the beginning for whatever hatches out of your 15 eggs in the incubator. If you use it, be sure to put the heat lamp where the chicks can go to another area if they get too warm. Also I would recommend that you use paper towels to line the bottom as they can be more easily changed in an aquarium than some sort of shavings or bedding material. Just keep the bottom clean regularly so droppings and scattered food don't build up.
One more thing, you should make some sort of guards for the inside corners of the aquarium to keep the chicks from piling up in the corners. Chicks trapped by the others in the corner of a square shaped brooder container could get squished, suffocated, or chilled. Making modifications to round the inside corners will help to prevent the piling up of chicks in corners. Think about shaping the inside corners like a car racetrack with curves in the corner turns.