I use some ordinary standard garden hose as an overflow outlet. how can i
fix something to the end of the hose, to act as a ' filter ' to prevent
dead
leaves etc from blocking it up? thanks

Not enough information. Where's the pump? ~ jan

there is no pump. its a small tank that some water flows into. instead of
just a hole (which would easily block with leaves) i have put 4 inches of
hose in to the tank outlet hole. now i need some arrangement over the hose
to act as a filter. thanks.

I'm still confused. Water is draining into the hose, so you want something
over the hose end? I assume with lots of surface area, because that is
where the clogging could occur? My suggestion is put the hose inside a mesh
basket http://users.owt.com/jjspond/koipond/photos/set1/09.jpg You wouldn't
have to use 2 as shown, screen door material on the open end should do it.

If no pump, how does the water flow? ~ jan

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(Do you know where your water quality is?)