1. What is the best way to purify hose-water?
If you mean remove the chlorine there are lots of bottled products. All are about the same imo.
2. Is this a good way to maintain ph?
Hydra Ph Buffer
Oh good, a chance to rant about companies...
I have a very low opinion of most companies selling these types of products. They say it's the greatest thing ever but won't say what it is. Too expensive to post that document on the web site??? Maybe they've hidden it some place, but who's got that kind of time. What are they hiding? At least they say it can take 2-3 weeks to buffer the pond. Thanks for that tiny bit of honesty.
Going by the picture, this is some crap slow release buffer. Could just be limestone for all I know. There are lots and lots of people who swear by slow release buffers like oyster shell, plaster of paris, limestone, you name it. Two issues, you need a lot and it's slow.
Get a heavy thunderstorm with a few inches of 2.0 pH rain water you've got ask yourself, is slow release good enough? The manufacturer says 2-3 weeks in normal water (although these things do release faster the lower the pH...but I'm ranting and don't want facts getting in the way). By that time the fish will just be bones caught in the skimmer. And it's expense, like $400 a year for a 5,000 gal pond depending on fish load.
Some people are able to predict the future and know for a fact they would never any kind of low pH rain problem. I unfortunately can't predict the future and don't have a Bluebird on my shoulder. So instead I adjust KH with baking soda and adjust GH with calcium chloride and Epson salts. Cheaper, but more importantly, it's an almost instant buffer and it can be measured. You know how much buffer you have. A pile of rocks or shells...is the pile large enough? No way to tell. Oyster shell and some other things are fine for micro minerals, but shouldn't be considered a buffer imo.
The reason for this long rant is that's one area where I think there's a difference between pond and aquariums..tons of really bad stuff available and a lot more bad info too. I know the aquarium hobby isn't perfect, but the low end pond hobby is pretty bad. I think it's because ponds are easier. Goldfish and Koi can take a lot of abuse and so these products don't cause that many direct deaths. In a small tank with more sensitive and expensive fish you pour in a product and 10 minutes later they're all dead people are going to get mad and start pointing fingers. With water gardens people just buy another $1 worth of feeders. High end Koi pond keepers don't even acknowledge these kinds of products.
3. How often & how many gallons are water changes?
24/7, Google "trickle water change". 10% per day is great, but most people (who do water changes) scale that back to maybe 10-25%. It depends on the type of pond you want to keep. Most popular method is no water changes, never test water, keep the pond for a few years and fill it in when the fish die.
Here's Andy Moo talking about trickle water changes starting at 7:20...I just like listening to him...
