Lacus in this case would be a small pool (lacus=lake) of water like the Lacus Curtius in the Forum (behind you in the Forum between the third and fourth of seven large brick structures. Both these areas never really drained even after the Forum was canalled. Likely there were springs where the water surfaced, either way they were very small and this one probably had a circular well structure build around it like the Lacus Curtius. This lacus was traditionally named after 'Servilius Tullius' the sixth King of Rome (578 BC). So it's very old if the tradition is true and later when Rome got water supplied by aqueducts it was very likely made into an aboveground ornamental fountain.
From ancient writings it's a guess that this is the location: (1) It could be seen when entering the Forum from the Vicus Jugarius. (2) It was at the beginning of the Vicus Jugarius. The Vicus Jugarius is that fenced-off ancient street between the Basilica Julia on the left of you and the Temple of Saturn. It also could have been just across the street where the Vicus Jugarius first enters the Forum and one source further puts it down the Vicus a little ways at the SW corner of the Basilica Julia.
The 'Oxford Archaeological Guide' puts it in this location but they also put the Arch of Tiberius in the wrong spot :-) . Well I've got to put it somewhere :-) and I like this location for three reasons: (1) It's a good high-profile location to display heads, (2) I could be wrong but this fountain seems to have disappeared from history after the Arch of Tiberius was built? and (3) The 'Oxford Archaeological Guide' puts it here.
This fountain is where the Dictator Sulla placed some of the heads of those whom he had ordered beheaded, mostly Senators. And if he had caught the young Julius Caesar, that is most likely where his head would have ended up. Sulla told Caesar to divorce his wife and marry another to form an alliance with him. Caesar refused and split town. So it was either for love or Caesar just didn't like to be ordered around, I like to think it was the former :-) . It's believed that this fountain was also used for displaying the heads of criminals even before Sulla's time.Next: #14.4: Arch of Tiberius