Didn't the boat get lighter when firing torpedoes?

Yes. In fact, one of the most important things to remember when torpedoes were fired was that the boat had suddenly gotten lighter. That mattered more when submerged because the sudden change in buoyancy can cause significant problems immediately. When firing torpedoes, the boat had suddenly gotten hundreds or thousands of pounds lighter. If they were firing six torpedoes from the forward tubes, the boat has discharged over 19,000 pounds. Some of that weight loss was made up from the water in the tubes that replaced the torpedo. The torpedomen might also be able to leave a drain valve open for bit to help compensate for the weight loss. In any case, the diving officers needed to be aware of the changes and may have needed to add weight in the trim tanks.  They certainly didn’t want to be light forward and broach.  They also don’t want to be light aft giving them a sudden down angle and an unplanned depth excursion.