Ceramic Hi oxygen air stones these are very good and give a really nice size bubble, they are available in 3 different sizes,
50mm x 25mm with 4mm nipple
50mm x 50mm with 5mm nipple
100mm x 50mm with 5mm nipple
when using the the 50mm and 100mm these have a 5mm nipple, therefore we recommend you warming the 4mm air line up first with hot water then it will fit over these.