Top Fin or Aqua Clear?

I would also say go with the aqua clear the foam filter can be rinsed and reused time and time again you don't have to keep buying cartridges
 
The AC's are great filters. But I always get at least the next step up from what they suggest for a tank. Decent water changes will help though if you do get the 30 gallon version. Just pick up some test kits as well.
 
Get the AquaClear. You have a lot more flexibility with what filter media you use with that filter. Just rinse that sponge out every month or so. Don't follow the manufacturer's suggestion and replace that sponge every three months like they say, it only needs to be replaced if it starts falling apart. My last one lasted for four years before it fell apart.
 
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