I have outdoor goldfish in a pond and I've rigged a home made siphon
for changing the water.

I have a length of old garden hose with shut off valves at both ends.

You can make one by cutting a hose to the length you need (great use
for old hoses with holes or splits) and buying a hose end replacement
to attach to the cut hose end. Gardening supply shut off valves and
hose end replacements are fairly cheap. Usually plastic, the threaded
hose end insert goes into the end of the hose with a collar that
screws down around the hose to hold the insert in place.

You need to have one end of the siphon hose with the screw fitting
that attaches to the faucet in order to fill it.

I fill the hose with water by attaching it to a faucet. After it
fills I screw a shut off valve on the end that came off of the
faucet. There has to be a closed shut off valve on the other end of
the hose so the water doesn't run out. After filling the hose I shut
off the fill end valve to hold the water in the hose.

I cut a hole in the bottom of an old spice container and ran the hose
through the container.

I lower the end with the spice container on it into the pond and
making sure that the intake end is higher than the outlet end, I open
the shutoff valve.

After opening the intake end valve, I snap the spice 'shaker' lid
back on the spice container, the part that has the shaker holes in
it. The holes are small enough to prevent fish or large tads from
being sucked into the hose.

Then I go turn on the outlet end of the hose to let the water flow,
and as long as that end of the hose is lower than the inlet end, the
water flows the right way and gets the siphon started.

The outlet end ports the water into an old ice chest so that just in
case there are any fry or tiny tads that are small enough to get
sucked through the shaker bottle, I can net them out and put them
back in the pond.

Additionally, I can set a hose attached to the faucet on low to
refill the pond at about the same rate of outflow, so the water
eventually runs clear at the outlet end.

I've got well water though, without chlorine or fluoride or other
toxic pollutants, and people on the city fluoridated and chlorinated
water supply may not be able to refill their pond without water
treatment or it can sicken the fish.