honestly a filter is not good enough only when it proves to be not good enough - I was able to run a surprisingly small internal filter on my 15g simply because the stocking allowed for it and my maintenance routine kept up. if you have no problem keeping things in good condition and the filter holds enough media to support your biological load, you don't need more. your stocking doesn't seem very heavy at all. that said, having more can't do harm.
I agree about aquaclear, they are the only HOB I will use - cheap, quiet, reliable, and more versatile with more media space then any other brand.