Coke bottle fluidised bed.

Take 2 2l coke bottles, 30cm of 22mm plastic overflow pipe.
Consume contents, remove label (fill with water at 60C, wash glue residue
with white spirit), and wash. (I assume your 2l coke bottles are like mine,
smooth outside, with one reinforcing ring round the top)

Hacksaw off the top of one, and cut the top off the bottle, through the
middle of the supporting ring, and trim so you end up with a hemispherical
bowl that fits tightly into the bottom of the bottle you've just removed it
from.

Now, glue (aquarium sealant) (possibly not actually needed, if you can
get it tightly fitting enough) the top of the bottle inside the bottom
of the bottle, so you end up with a smooth hemispherical bottom.
Now, cut the top off the other bottle, below the supporting
ring, so that it fits over the top of the first bottle, with the supporting
ring in the same place.
Taking some scissors, cut 5 or 6 large holes in the top of the bottle.

Fill the bottom half with washed sand.

This ends up (if you can get the wrinkles out, which I found possible)
with a bottle looking the same shape as the original one, but with a
hemispherical false bottom.


Cut the bottom off the second bottle, and now using a hot wire/soldering iron/
dremel, punch holes in about half the area of the bottom.
Cut a 22mm circular hole in the bottom. (getting it to 15mm, then heating
with a cigarrette lighter and pushing the overflow pipe through works)

Glue this over the top of the bottle.

The overflow pipe has the end cut like a hole-saw, with 6 or so 'teeth',
to encourage swirling.
This goes all the way down to the bottom of the bottle.

Now, take a hosepipe, and outside, slowly increase the flow through the
filter, until you wash about half the sand out over the period of 10 mins.
(to grade the sand, and get rid of all the fine stuff)

Attach to the output of your existing filter.
You may need to significantly reduce the flow - my fluval 4 nearly blew all
the sand out.