Had you checked the pH before you did the water change? Where do you get your water from? Have you checked the water from your source? What do you use to check the pH?

A reading of 9 is unusually high and suggests there's been some form of contamination somewhere along the line. Decaying vegetation can acidify water but cleaning debris out shouldn't have a particularly dramatic effect. People do use baking soda to raise pH in water that's too acidic, definitely not needed in your situation.

Well, if your pond is Really only 35 gallons then it is WAY too small for koi. My guess is that it is bigger than you said.

Re pH: baking soda is used because it will bring regular water to pH 8.3. That is a good value for fish. It will raise low pH values up and high ones down; very convenient. Actually, pH 9 is not horrible, and as tropicbreezent is suggesting, your measurement may be off anyway. But if your pond is as small as stated, then it would be easier for a minor screw up to cause some trouble. If it is really pH 9 then some baking soda would help.

Thank you all so much for your help. The pond IS only 35 gallons as it is a prefab one. I started out small but do intend to make a new one that will accommodate the koi. The water source was the same as I have always used. All other levels are great. The Ph is starting to come down. I also tested it in the morning and it was around 8. I do use a drop system to check not strips. I am learning as I go and have learned something from this episode; check the source water before it goes into the pond even if it is where I have previously gotten it from. Also, I will not leave dying plants in the pond too long. That really made a mess. The new pond will be about 250 gallons. Do you have any suggestions for a pump and filtering system for this size? Currently I don't have a filtering system. I just do periodic water changes and use the siphon tube to clean out debris. I know that will NOT work in a larger pond. The current pump I have has a small UV light but it keeps blowing out. Is there one you would suggest to control algae. I hesitate to use a product for that. I did use the baking soda, just a pinch. I think that is working. So again, that you for responding.

Yes, pH is almost always lower in the morning and a value of 8 means everything was and is fine. You probably don't need to worry about it. Since you will be creating a new pond, you might want to consider my stream [LINKED HERE[(https://www.houzz.com/discussions/i-love-my-stream-2-dsvw-vd~1589581) approach. I think even a very short one would be a good filter.

You should probably search here on 'string algae' as well because many of us believe it is beneficial, reasonably attractive, and blocks single cell algae.

I dont think 8-9 is anyting to worry about - s Some of us in Midwest w/ limestone aquifers have high pH no's naturally and goldies/koi do fine in higher pH (which btw is a base and not acidic).

However you are way, way, way overcrowded- Im so relieved to hear you are expanding your pond. it is a total myth that fish grow only large enough to fit their tank - more like they cant grow properly and likely to die an early death. Sorry to be blunt but there it is. The recommendation for koi is 1000 gallons for the first fish and 500 ea for the others. and a minimum depth of 3' (4 is better) . Your pond may look sparse at first but remember these are fish that will soon get 24-30 inches long and you want them to be healthy and active. You might want to check out koi specifc websites like koiphen - where you will find more expert koi advice than youll find here. They are a more demanding animal to care for because of their size but so worth it. Best to you and your fishies!

EDTA: I just saw your post re: enlarging to 250 gals - that will be ok for now but you will probably want to find new homes for the koi at some point. There are some comets and shubunkins (goldifish) now being bred for ponds with bold patterns and long flowing fins - some resembling koi - you might have to go to a pond specialty place for those. Difference being they only get to 10-12 inches and not 2-3 times that!

(Sorry I seem to have missed/misread some posts in this thread.)

Hey some of the above is not at all accurate - a pH of 7 is average so 8
or 9 really is no big deal. Higher than 7 is a base, while under 7 is
acid (with 0 beingmost acidic). Some of us in Midwest w/ limestone
aquifers have high pH no's naturally and goldies/koi do fine in higher
pH.4.

Current Resident: What are you talking about? I am the only one who said anything about pH, so you must be referring to something I said. However, as far as I can tell, you then repeated what I had already said. We both said pH 9 was probably OK. I didn't say anything about acidic values.

BTW, there is no such thing as 'average' in pH values. We do think of a pH of 7 as 'neutral' but it actually is just the acidity of Pure Water. Water naturally has a few free hydrogen (H+) ions (and OH ions nearby too) in addition to tons of water molecules. Specifically, it has 0.0000001 moles of those hydrogen ions (10 to the minus 7 molar or pH 7. A pH of 6 or 8 will have 10 times more free Hydrogen or Hydroxide (OH) ions specifically. At pH 5 or 9 it is 100 fold more or less hydrogen ions. So pH is not a linear scale. The likely reason that fish could tolerate either is because similar to the chemicals around them, they also have chemicals including buffers that can neutralize too many or few hydrogen ions.

Buffering is what goes on with sodium bicarbonate. Limestone does the same thing (but pH closer to 10 I think). With bicarbonate, it will react with extra hydrogen ions until the pH is 8.3 and also bring too many OH ions lower until they are at pH 8.3.

Sorry Chas! not sure why I missed your post - yours is good info and I learned some stuff I didnt know. Iwas responding to one above seeming to suggest that higher nos. meant its acidic (?) Anyway I will correct my post so as to not confuse future readers -thx