The Rybníček u Hořan NNM (Fishpond near Hořany) was declared in 1985 on an area of 2ha lies in the Středočeský kraj - Central Bohemian Region and is managed by the Blaník Protected Landscape Area Administration. The monument is the only locality in the Czech Republic where the critically endangered opposite-leaved pondweed (Groenlandia densa) occurs naturally. The protected area covers the pond itself, but also a valley, spring and forest stands.
The opposite-leaved pondweed is a richly branched and completely submerged aquatic plant, which was always relatively rare in this country. During the 20th century many suitable biotopes have been lost due to dredging of ditches and channels, regulation, forcing streams into pipes, small streams drying up and especially water pollution.
On the pond surface we can also observe thread-leaved water crowfoot (Batrachium trichophyllum) and common duckweed (Lemna minor). The banks are fringed with plicate sweet-grass (Glyceria notata), hard rush (Juncus inflexus) and celery-leaved buttercup (Ranunculus scleratus). An ash-alder wood has developed above the pond, with ash (Fraxinus excelsior), bird cherry (Padus avium) and herbs in the undergrowth.
The territory is negatively affected by runoff from the surrounding agricultural lands, which causes eutrophication of the pond water and promotes the growth of algae and worsens the light and oxygen regimes for the opposite-leaved pondweed. The population of this pondweed at Rybníček u Hořan NNM has been observed continuously since 1976 and a reduction in their numbers was first recorded two years later. This was due to sedimentation and a reduction in the ponds area. Since 1980 small-scale interventions have been carried out on the pond, including partial sediment removal and repairs to parts of the dam wall. In 1995-96 the pond was completely reconstructed and a dry polder was built further upstream in the valley, which should serve to contain flood waters and restrict the sedimentation of the actual fishpond.
Opposite-leaved pondweeds from the Rybníček u Hořan NNM have been grown at The Silva Tarouca Research Institute for Landscape and Ornamental Gardening in Průhonice since 1995. Sufficient quantities of the pondweed have been cultivated and are now held at the institute as a gene bank.
The territory is accessible by way of a field track leading from the Mozkovice – Hořany road. The aquatic vegetation can be seen from the dam of the fishpond. It is forbidden to enter the water or collect plants or their parts.