New owner of Koi pond/maintenance

Question
We have recently purchased a home with a Koi pond.  It has approximately 12 generations of fish (information from previous owners) living there.  It has very large plants inside the pond (lilies/cat-tails/other). A week ago, 1 large fish and two small fish died.  We pulled one large plant box out of the pond to clear more area for swimming.  We are having very hot weather and the tree adjacent to the pond area is loosing leaves already.  It is a very large tree and will loose all of the leaves soon.  Do we need to cover the pond to prevent the leaves from falling into the pond?  And are so many plants (the plants almost fill the pond area) good for the fish?  We live in the upper part of Oklahoma (north of Tulsa about 50 miles).  We have never owned fish before and are needing a place to seek help with problems.  
We would appreciate any advice and thank you in advance.

Ignorant owners,
Susan and Jerry Smith

Answer
Hello Susan and Jerry!
Sorry to hear you are losing fish! Most of the time fish loss at this time of year is due to oxygen depletion in the pond. Make sure you have plenty of aeration (fountain and/or waterfall). This must run 24/7 this time of year.
The fish do, of course, need room to swim. Also, be sure there is adequate filtration to take care of fish waste.
We have some excellent (short and easy to read) free e-books about pond and fish care on our website. They may help you understand the workings of the pond.
Just go to www.pondmarket.com and click on any of the "e-book" links.
Brigitte
www.pondmarket.com