Question on Changing Filter Types

Tynk wrote:
« Grìffîñ » wrote:
Tynk wrote:
I love sponge filters!
They are a lifesaver (literally) for fry.
I have even made my own for Betta fry tanks out of pvc pipe and "clean"
sponges. By clean I mean not used, and NO chemicals in them. Your
average dish sponge has toxic chemicals in it and cannot be used.
I made my own because I got tired of the ones in the shops you can buy
that tend to float up.
They're also a great thing to "seed" a new tank and add the benificial
bacteria to it.

Please tell me what type of sponges I need to look for in the stores.

I will often just buy a replacement sponge at a local shop and then cut
it to fit the filter.
Shops seem to have a lot of different sizes these days to choose from.
There weren't so many around when I made my own, so it's so much easier
to find new sponges.
Especially these days with the anti mildew chemicals that are put in a
lot of sponges you find at the hardware / grocery stores.

I gather that I'm going to have to SWAG my own sponge design.
(scientific wild ass guess)