I need suggestions, please.

[Alberta_madcow]

When using hang on back filter how often do you change the replaceable filter? the one with carbon stuff in it? I know not to touch the bio filter.

[intarsiabox]

The carbon has a very limited effective lifespan. After a week it does very little but still works to some degree for another couple of weeks. I would personally change the filter every 3-4 weeks but rinse it out with each water change. If I understand you correctly you are using the type of filter pad that has the carbon and filter pad as a single disposable unit. If you have one with a sponge and carbon separate then just replace the carbon and rinse out the sponge as the sponge will last a very long time.

[Alberta_madcow]

Thank you!!

[reefy]

spike-fin goby is the best goby for a "stablished" nano IMO