This is the part that goes at the end of the stainless steel rods underneath the sponges. It can occasionally break if excessive force is used when backwashing the filter.
There are two sizes available but this one is specifically for the following models
OASE Filtoclear 3000, 6000, 11000 and 15000.
It will NOT fit the 12000, 16000 or 30000 models