How to get there:
Tram 27
has a station called "Am faulen See". There you get out and walk west
for five minutes. You can catch Tram 27 at S-Bahn station
"Friedrichsfelde Ost" (S5/S7/S75) or U-Bahn station "Tierpark" (U5).
The one going in the direction of "Pasedagplatz" is the right one.
The lake is not shown on the google map.