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Overflow boxes and freshwater

Mark asks: "Does anyone have any ideas or tips to keeping fish out of
your filter? I glued a screen around the teeth on the overflow box, to
keep the fish out. The problem now is that the screen clogs with gunk and
plant matter daily and needs to be cleaned<snip>"

I have used two different brands of overflow box as well as made them
myself. My observations are that the narrower slots, about 3 mm wide, are a
better design for dealing with this problem. When using boxes with wider
slots, I've had to use a screen of some type as you have done. The
objective is to use a screen with a mesh size that will be just small
enough to reject  whatever you don't want to go through it. This delays the
clogging process as long as possible (and the related problem of water
level in the sump). I'm not sure what kind of screen you are using, but if
its window screen, its too fine mesh and will clog too fast. For me, the
best source for screen material with various mesh size has been the local
supermarket. The produce section has fruits and vegetables packaged in
plastic netting material of several different kinds. I usually cut a 2"
wide piece and stretch it around the "gate" of the overflow box and use the
plastic screw that attaches the box to the prefilter to pin the ends
together. Another method is to use a rubber band around the box to hold it
in place.

Of course, you don't want any large debris suspended in the water to go
into the biochamber of the trickle filter and this includes fish, shrimp,
and snails.  The next step in the mechanical filtration process is the
sponge cylinder that usually comes with the prefilter. This is where I find
out if the netting I have used on the skimmer box has a small enough mesh
size. I check this location daily. It usually contains only snails unless a
fish has jumped over the gate.

On the issue of a backflow situation: Most people just drill a small hole
in the return line at, or slightly above, the waterline. This spouts a
small jet of water while the system is running normally. When a backflow
starts to happen, the small hole then sucks in air which effectively breaks
the siphon and prevents the sump from overflowing. To make sure you drill
the hole in the right place, carefully mark the return line (hose, tube, or
pipe) at the waterline while the system is running normally. An alternative
method is to use a check valve in the return line.
-- Eric