green water

[b.potter]

is it safe to flush out a fishpond with tapwater to clear green water, with the fish still in there

[wildwood]

No. They will almost certainly suffer from shock and could very easily turn belly-up.

I am not a fan of adding chemicals to fish water, but you can get remedies for green water from a good pet shop.

If you have a 'freeflow' shaped or rustic pond you may wish to consider making some soft of frame with 1/2' wire netting so that half of the bottom of the pond is covered with it 6" high. Then add as much daphnia as you can get/afford. A small portion on the south side of the pond partitioned off right up to the top is even better.
Use plastic covered wire if you can, it lasts longer and is not as noticable.
The daphnia will make short work of the algae and some will survive in the partitioned section to breed and continue to keep the water clean.
A big advantage of this method is that the fish will have the occasional live snack which will benefit them greatly.
Disadvantage is that unless the pond is natural, the wirenetting will be easily seen. Crunching the netting and then roughly straightening it before installing will make it less conspicious.

[showcatbenga]

Because we have chlorinated mains tap water in most parts of the UK, you need to dechlorinate tap water before you use it for your goldfish - pond or aquarium. Chlorine is poisonous to fish & while they may survive they will be distressed & may then fall ill. Dechlorinating can be done by drawing off the water required & allowing it to sit for a day(5gallon bucket) (larger the volume=longer standing time). Or you can get a chemical dechlorinator at most good pet shops or pond centre. Or use clean rainwater. I suggest that if you feel the pond needs a thorough clean that you could use a large container of conditioned water i.e. a very clean dustbin, Add an airpump, then as you bail out the pond catch up your fish into the airated container. This allows you to check the health of all your fish. You can then finish cleaning your pond, re-plant, refill with dechlorinated water & allow it to settle before returning the fish a few days later - (that's why you need the airpump to keep the fish well during this time.)
You will lose any natural pond life by radically cleaning a pond in this way & may then have a problem with blanket weed due to nitrates in the new water. Seek further advice if this seems compicated at a 'pond' centre.