I've used the first and third displayed in this link...
http://pet-supplies.drsfostersmith.com/search?af=type%3Aproduct&view=grid&w=prefilter&visitorID=&cartcount=0&wishcount=0&subtotal=0.00
But I didn't pay anywhere near that price... Wow...
Or you can take any old sponge... Saturate it... freeze it... drill a hole in it (while frozen)... let it thaw.. and you have a perfectly round hole in a sponge... which is all a "prefilter" really is...
http://pet-supplies.drsfostersmith.com/search?af=type%3Aproduct&view=grid&w=prefilter&visitorID=&cartcount=0&wishcount=0&subtotal=0.00
But I didn't pay anywhere near that price... Wow...
Or you can take any old sponge... Saturate it... freeze it... drill a hole in it (while frozen)... let it thaw.. and you have a perfectly round hole in a sponge... which is all a "prefilter" really is...