There are too many variables to go into great detail.
But if you are suffering from high pH 8.5 or higher then the safest easiest solution to your problems is to treat your plants for the symptoms they are having. It is a common problem that plagues land plants as well as pond plants. you plants will tend to start to yellow and maybe even turn red. May also show yellow veining. Chances are this is caused by the high pH of the water. High pH makes the minerals and nutrients in the water unusable by the plants. Basically you could load your pond full of fertilizer and the plants would still starve.
If it happens to be self induced (you added something to the water to raise the ph) then the best course of action would be to treat the plants for the symptom for the short term. And let the pH naturally return to your normal range. There are several products out there that are hign in iron. Fertilome makes a liquid iron supplement that is used as a foliar spray giving the needed iron directly through the leaves. Another option is to give up on the pond fertilizer tabs. Fertilizer is fertilzer. And to instead use a reg garden fertilzer high in iron. Ironite makes both a regular fertilzer high in iron (around 2%) and a granular iron supplement (4%).
If you are on well water much as I am you have to make a decision. Some plants no matter how hard you try, will not tollerate high pH. Personally I know water lettuce and water hyacith will not live in pH above 8.5. Using the above methods you can keep them but it requires alot of extra work. Bi-weekly baths in miracle grow followed by iron foliar spray. They will stay green and live but will not flourish like they should. I will not be replacing them next year in search of other alternatives.
I have had very good luck with any hardy lily, pickerel, and primrose creeper. This list will expand as I have new additions to the pond. Oh and algae. It lives anywhere!
I dont want to get into a pH discusion but it is better for the eco system of your pond to allow things to adapt to what you have then to try to make your ecosystem "perfect" Trying to lower the pH can make the ph "crash" and drop drastically killing all your ponds living things.
On a side note for any pond plant that is in need of a little pick me up. 1 Tbps of miracle grow to 1 gallon of water. Place and mix well into a tub. Pull plants form the pond and soak in solution for 24hrs. Its best to start in the morning and allowing to soak overnight. Pull plants rinse well and put back into pond. 3-5 days your plants will look better them ever. You can use excess solutiuon to water your land plants, garden, grass or whatever you wish.