How often should I change filter bags?

[dismayed]

I have a bag of activated carbon and another bag of phosguard in my tank. I've been changing them out once a month... is that often enough?

I've also been thinking of getting a poly filter or some kind of nitrate filter and putting it in my tank, so same question for that as well. Thanks.

[demonsp]

Once a month is fine. Just another tool for use to control the water quality.
You want to add more goodies. Do you need these?Do you know your water readings? Is this a reef tank? Type of stock?

[moo0o]

once a month for carbon is fine