Freshwater

Undergravel filters have one or more perforated plastic base plates that sit
on the bottom of the aquarium with a gap between the bottom of the plate
and the bottom of the tank. The base plates have holes for the insertion of
plastic uplift tubes (which allow air to flow up from beneath the filter plate)
containing an airstone which connects to an air pump.
Airstones come in many shapes including globe, circular, and rectangular. An
airstone can be made out of many different materials including porous wood,
fused glass, plastic, and refined ceramic. An air-line tube attached to a pump
forces air into the stone which releases it in the form of unified bubbles for
greater aeration.
An undergravel filter pulls water down through the gravel and the slots in the
plates and returns it to the tank via the airlift (uplift) tubes. During this
process ammonia is broken down as the water passes over a colony of beneficial
bacteria living on the substrate’s surface and in the space beneath the
plates. Debris is trapped along the substrate bed, making it easy to vacuum
away. Periodic vacuuming is a must with this type of system to keep the bed
from becoming clogged. (If you don’t vacuum, eventually your substrate bed
looks as if a mudslide hit it.)
Undergravel filters are one of the oldest systems around; many hobbyists do
not use them much anymore. We still believe they are a good building block for
aquarium filtration. Many pre-packaged kits come with undergravel filters
included. As long as you have it as part of the package, you might as well use it.
Undergravel filters are great for systems that do not have big rocks or decorations
to block large sections of the gravel bed. Blocked plates create dead
spots on the filter. A regular, store-packaged gravel substrate is best for this
system — because smaller substrates will fall through, and larger ones can
hide big chunks of debris (your lost golf balls, your child’s hidden leftovers).
Set the undergravel filter in place before adding the substrate (usually
gravel).
Sponge filters
Chapter 6: Equipment and Other Technical Stuff
A sponge filter provides biological filtration. This type of filter is simple in
design and, when attached to an air pump, draws aquarium water though a
large sponge that acts as a medium for bacteria to gather on. Sponge filters
are good to use in quarantine and hospital tanks because they have no chemical
filtration that can ruin the effectiveness of medications you may be using.
Sponge filters are also useful in fry tanks and aquariums with small fish
because they eliminate the danger of youngsters getting sucked up into
standard filtration units. However, sponge filters only take care of biological
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