We've just had two chicks hatch - one is not far off 48 hours, the other is about 3 hours old. When the second chick dried out we put it in the brooder and the older one, who has become extremely active, started pecking at it, and it looks quite rough. Is this normal...(older chick pictured below).
No higher, and no longer than a week. Once they are thoroughly dry and active, I'd drop it back a bit. Sometimes if they are lethargic it is because it is a bit warm in the brooder. There is also the problem of ventilation. Something like a fish tank may not allow the ammonia from their poop out well enough. If you can smell ammonia, it is already too high concentration.
It's best to determine temp by their behavior, which means having a large enough brooder that it has a cooler and a warmer end, so they can choose. This should also help the ventilation problem. I raised two groups in my house (I'll never do it indoors again) in a huge brooder made of appliance cartons. I was amazed that even if the temp below the heat lamp was less than recommended, they would always go to the cooler end. Later I raised a group in my coop with a heat lamp which really didn't keep their area all that warm. Outdoor temps were 16C at night. At 3 weeks they wew moving as far from the heat lamp as they could get, almost a meter away, so I just turned it off. They grew up healthy and strong. In other words, except for the first day or two after hatch, or while recovering from shipping stress, I think we often overheat them.
This happens quite often when serial hatching chicks or hatching them off one at a time. The first chick sees itself, (at least in its own mind) at the top of the pecking order, and now come heck or high water the oldest chick will yield to no chick. It doesn't matter if the subsequent chick(s) is pushing its sibling for supremacy or not. To a poorly imprinted chick the very presence of another chick is enough to bring on this behavior. It is a HUGE part of chicken DNA and outside of genetic modification I don't know now to stop it. But if you'll hatch a whole clutch worth of eggs at once, the chicks hopefully will imprint on their brothers and sisters and not become such pains in the you know where for you.