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Different strokes for different folks I guess, and isn't it grand that we have so many filtration devices to chose from so that we can be sure to find the one that suits our tank, our personal needs and the requirements of the fish.

Personally I'd like to see the UGF marketted as specialist equiptment, it would save a lot of tears all round.

For animals, the entire universe has been neatly divided into things to (a) mate with, (b) eat, (c) run away from, and (d) rocks. - Terry Pratchett

In my opinion, inalmost every case a hang-on-back filter is better, especially one with a bio-wheel. The best thing about having the bio-wheel is that you know exactly where all your good bacteria are cultured, so so long as you take care of that wheel, you don't have to worry about re-cycling the tank.

There was a time that the undergravel filters were the best technology out there, but today that is just not true. Here are some of the drawbacks as I see them: the gravel bed, since water has to flow through the bed to be filtered, may develop channels, also known as ratholes, through the bed. This is bad since if the water is not running into the gravel too often, it is effectively getting not filtered. And that will happen since the water will take the path of least resistance, which is the rathole. Next, since the bacteria are under water, the water has to be well-oxygenated... the good bacteria need oxygen to work. With a bio-wheel, the bacteria are exposed right to the air, which gives them at least 1000 more oxygen to do their work with. Another thing I like about the biowheel filters is that you can immediately see if it is working correctly, that you can see is the flow rate of water is faster enough, you can see how often the wheel is spinning. And, again to fix this, you just take the wheel off, float it in the tank, and then clean out the filter itself. You know the bacteria are safe on the wheel, and you can clean that filter and pump and the filter pads and anything in the filter as much as you want without worry about a cycle. Comparatively, since with an undergravel filter the entire bed is the filter, you actually have a limit on how much you can clean the tank. Clean too much, and you may cause a mini-cycle. With the bio-wheel, so long as you match the pH, hardness, and temp of the new water with the tank water, you can actually do as large a water change and cleaning that you want. This could be good if an emergency arises.

Ultimately, you can never use sand with an undergravel filter, and very few plants do well -- they don't appreciate the currents on the roots. And, there are just better, easier technology for use today. This is not to say that UG filters don't work, obviously time and experience has proven that they do, it is just that starting a brand new tank today I would go with a bio-wheel HOB filter in virtually every case.