Dying Fish

[Blubble37]

probably some type of ion exchange resin methinks...

 

[ShinySideUp]

Juwel do a green coloured sponge they call a nitrate filter which should be changed on a regular basis. I think it is aimed at people who don't change their water as often as they should so the nitrate sponge does the job for them, but only for a time. But then if you aren't doing water changes you are hardly going to change a filter that removes the need to do water changes so there isn't really much point having them.

PS Not saying this is you Aimsley, just remarking on nitrate filters generally