The Kistnerweiher is designated as a nature reserve. The walking around the pond economic and hiking trails may be used by cyclists, but the area around the actual waters is closed (see also the appropriate signage) - the pond should not be used for swimming. For the nature reserve there is a year-round ban on access. There are also checks carried out here, so that in case of fines can be expected. But even the view of the lake is a pleasure! Who wants to swim can make a detour to the nearby lake Schlicht.
So far as I know you can enter the area on the roads and ride on bicycles. Absolute ban on swimming, which is also controlled. Of course, for the benefit of nature no noise and garbage cause, but I take that for granted in a nature reserve.