It depends upon your clientele in the tank.
Sponge filter - especially useful for really small fry. They don't get
sucked in, but they can browse for rotifers on the surface. Pretty good
biological filter for its size and of course today there are quite a variety
of sponge filters. Go for as much unobstructed surface area as you can.
box (corner) filter - good as an all purpose filter and for water movement.
I've used one with large stones in it not for filtration but just for water
movement.
Weekly partial water changes still can't be beat!
undergravel filter - great biological filter and the hobby seemed to have
figured out about them before the techical people really did. :)
However, if (like me) you are not a militant water changer and you do not
weekly gravel vac all of the goop out from under that u.g. filter, you are
setting up a biological time bomb. Too often, within a year, tanks with
uncleaned u.g. filters will sour and everything in the tank will die.
If you are a snail person with a failing u.g. filter, you may have a small
window of warning because the snails may either come out of the gravel and
sometimes even try to vacate the tank or they will die off quite quickly.
At that point get the fish out of the tank immediately!
If you have to call in dead to work, oh well. One must have their
priorities. :)
All the best!
Scott
The difference between a newbie aquarist and an old timer is that the newbie
didn't know what to do before the disaster.
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