I'm not familiar with the Topfin filters, but, if you can, take some the media from it and add it to the new filter to help "jump start" it. dcutl002's suggestion is also a good one.
I run an AC
50 and AC20 on my 20-long, both set to about half-flow. That's because I wanted extra biological filtration and good flow at both ends. A single AC
30 would be fine, too. Aquaclears overall are great filters.
Now open: "JJ&J's Seafood Buffet
" 20g long planted freshwater - Sunset Fire Platy (x5 adults, many young (argh)),Neon Tetras (x11), x3 amano shrimp, 40 lbs. Eco-Complete, AC 50 + AC 20, Finnex Planted+ 8 hours per day, Seachem ferts.