Recently I acquired a "pre-owned" 20 gal aquarium with a Whisper 20 filter, w/o a UGF, new gravel/river rock for 2 young Moors (bodies appx. 1.5 inches in length). I want to upgrade the filter, even though it's functioning normally, to ensure the tank is receiving good biological filtration. I do not plan to keep live plants, though may change my mind once everything is in equilibrium.

I read the posts under "What is your opinion on filters for 20 gal tank" and I went to the pet shop to investigate the Emperor 280 Filter (mentioned by Jetman73). It seems like an easy-to-use, quiet, effective filter with an appropriate flow rate, but I wanted to get some opinions from people with more experience before I go ahead and buy it. Thanks for any comments or suggestions!

The other Hang on filter that I like is Peguin Bio wheel filters they are a good one.

The Emporer is the big brother to the Penguin. I've got a
penguin 330 on one of my 20's and it works super, so a emporer
280 should do what you need it to.

Here's a price comparison if your interested. From petsmart.com
penguin biowheel 330 $24.99 emporer 280 $37.99. If you walk into
the store double those prices though.

The only reason I like the emporers over the penguins is because of the spray bar on the biowheel. ( I am not sure if the 330's have it and someone please correct if I am wrong). In my personal experience the biowheels that lack a spray bar get gunked up pretty quick and restrict the water flow. They also need too be cleaned frequently where as the ones with the spray bar never seem too accumulate excess gunk. With my emporer 400 I just let them float in the tank water during maintenance time and just pop them back when I am done. Never had a problem.
This is not a knock on the penguins since I think they are good filters but this is the reason I prefer emporers. They also have the 280 on sale at drfosterandsmith for $34.

Thanks everyone for your input! Since the whole concept of a biowheel is new to me I appreciate the comments. Yes, I've been out of the fish-keeping for a few years and wasn't exactly savvy before that. :-)

I will probably get an Emperor because I am concerned about noise level/biowheel clogs. Thanks again

Just wanted too let you know that they can be a little noisy in the beginning but after a few weeks of use they become extremely quiet. Another good point about the emporers is they come with a spare media cartridge so you can add exactly what you want. Right now I am currently using some coarse material (ehfifix) before the regular cartridges too extend the regular cartridges life even longer. Although I am always tinkering around too find the perfect setup for them.

Cool! Glad to know ahead of time what to expect with noise (my sweetie is not too keen on the noise of aquarium filter/pump).