These direct replacement cartridges work great, no question about it. However, there are cheaper alternatives... Take the filter media off of an old cartridge and keep the plastic piece. Purchase a bag of filter material (floss or other type) and cut to match the plastic. Just hold the two together and insert into your bio-wheel filter. The plastic keeps the filter material from sagging in the filter, and it doesn't need to be attached. If you want carbon, get a mesh bag and drop it in one of the media baskets within the filter. Little more work than the direct replacements, but a hugh cost savings. I can get about 20 filter replacements out of a $10 bag of filter floss.

Pros: Works great and bundled with active carbon!

Cons: Expensive!

I have two Penguin 350 Filters that I run on two of my tanks with Bottom Dwelling Breeding Bristlenose.
What I like about these filters is that you can buy multiples in a pk.,and that they really hold up well for the duration of time that I use them.It's nice to have it ready to pop right into the filter as it doesn't get any easier than that.I find that if I rinse them on occasion while I'm doing a water change,that the filter cartridges will get a lot longer life,and that the filter will continue to do a great job.I definitely think that I get my money's worth from them!

Pros: one deal pkg includes all you need

Cons: none

Yes, as the previous review said, there are cheaper alternatives. When I had 14 aquariums in my house I found ways of making a lot of things myself. I even got very good at it. But, due to a busy life, my number of aquariums has dropped back down to 2-3. I am also back to being able to spend more money and less time on maintenance. This is a great product that is very easy to change as needed (generally should be at least once a month) and it works wonderfully.

Pros: convenient

Cons: not cheap

I realize there are cheaper alternatives, but these have always worked for us and are easy to use. They last about 3 weeks under normal use

Pros: none

Cons: none

I had a couple of the Penguin brand filters and used these cartridges in them. They were very effective keeping the water clean.

Pros: I used these

Cons: none