use the speed setting on your TDC. Slow(long range) med(medieum range) and fast(short range). But its a bad idea to fire a fish at a target more then 4-5,000 meters away anyway. Unless you are using eletric fish. Then its one speed one range.
The secret is to get in close to your target when attacking submerged. Targets at a good distance i.e +1000 meters away can be harder to hit, as any last second course deviation by the target will guarantee a miss, and a longer torpedo run can increase the likely hood of the torpedo failing by premature detonation, which can happen quite often even at shorter torpedo runs.
Personally I always engage targets within the 1000m distance, if I can help it I prefer to attack from a range between the arming distance of 300m to a max distance of 500m (I always add an extra 50 meters to the arming distance of 300m so in effect I engage a targets of no less than 350 meters).