I got several canisters. The one on my planted tank is a Zoo Med nano, 30 gallon. It has one course sponge in one compartment, biorings in another, and pinky filter floss wrapping polyfil in in the last chamber. I don't keep carbon in this filter ever because it can take out what's in my ferts. Carbon on Planted tanks isn't a super big no no, but most people just don't do it.
The other two canister are on my cichlid tank.
My first is a penn-plax cascade 1000, top compartment is polyfil and pinky filter floss, next is biorings with another sponge. Last chamber has seachem purigen with a very course sponge.
My second is the mother of all filters, A fluval fx 6. It has 6 filtration stages, 4 of them are various sponges fine through course. I also use carbon with polyfil and biorings in the other 2 stages.
Polyfil is kind of a secret weapon for a lot a canister users. It's cheap, comes in bulk, and does better filtration work then most fine sponges polishing your return water. Also you just throw it away every time you clean your filter. Fine sponges get clogged easily and really need a more rigorous cleaning then your other sponges if you want to keep them at peak performance. So some just ditch'em for pure polyfil like I did in my penn-plax. I've kept the one in my fluval because so far it has taken my abuse when I go to clean it but I'm sure it'll go the way of the do do one day. Another filter upgrade is purigen over carbon, it can last longer but that isn't the case for me. But my cichlids are super dirty fish in an aquarium that is purposely overstock to minimize aggression. Lastly my pinky filter floss is I treat much like my polyfil. It catches a lot but will withstand the abuse of a cleaning. I normally wash it then throw it back for one more round in the filter then ditch it. Pinky filter in my experience is one of the best filter floss products on the market and you get it in bulk. Pinky floss can be used in any filtration system to give it a bit of an extra boost to filtration and I use it practically in all my tanks.
Another product you might also want in a canister is Chemi-pure blue, I haven't tried it myself but I've only heard great things about it. Definitely a product for a non-planted tank but it's a specialized carbon with resins. I'm going to try it one day, I just can't get my hands on the stuff in less I buy it online.
There is no such thing as ‘Rescuing’ a betta or any fish for that matter from any LFS. Be it petsmart, Petco, or exotic and aquatic’s down the street. You are not ‘rescuing’ anything, you are buying a fish. It doesn’t matter if that fish has fin rot and you are saving it from dying a slow death, you are still supporting an industry that thinks it’s okay for these fish to live in small pint size bowls.