I've got a couple of 3s and a 4 - great filters. Since you probably have the sponges in the bottom tray, I would stick it between the layers of sponge.
And a helpful tip - if you use carbon, put that in the top tray that way it's easier to access. If you put it in the middle tray like you are "supposed" to do, then it's more work changing it because you'll have to remove the top tray to access it. Were it me and I wanted to use carbon, I would get a small HOB to keep it in to make it super easy to change. Unless you have a lot of plant matter being sucked up by the filter, it can go for a long time between cleanings. I clean mine twice a year, and I'm a few months late on the summer cleaning. That's the only time I open them up.
I got the whole set up for free and was just gonna run what was in it but I think I need to fill it with more bio balls or something. What do you guys use or recommend. I finally found a good lfs in my area so I can get whatever I need to
As long as your water tests show 0 ammonia and 0 nitrites, you've got plenty of media from a bioload perspective. If you have particulates in the water and water clarity is a problem, then you can add finer media to collect that. If water clarity is good, then you don't need to worry about that.
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