Question
Ok... This is a good one!

I have looked after my pond for the past six years.  It's about 650 gal.  Last year I noticed my koi dying one by one on some days two or three, the whole process happened over a couple weeks. They grew up with two plecos (Which were about 7 inches at the time of the Koi death) The Koi were about 16 inches when they died.  When the first couple died I was not on the island (I live in the Caribbean) so I could not investigate.  When I returned home another died, so I decided to inspect it. They were no external bruises or cuts.  All the scales were intact the fish was a lil swollen in the stomach.  I decided to cut it open to see what I would find.  To my supprise it was full of eggs.  To make a long story short maybe 80% of the remainded when they died were full of eggs.  so I decided tro start over.  I purchased some koi...again to my supprise they died. So I was a saddened and decided to change from Koi.  I purchased some tinfoil barbs... they too died.  Figuring that it was something in the water I did a total water change and I went with some more koi..they died.  Did another water change and went with black moore, ghost knife and mollies.  Gold fish died...other fish survived as well as pleco.  added some silver dollars...they live.  add a shark, angelica and anglefish. they are living  added some commets...they died  added clown loaches and Haliries...they are living  added some tinfoil babrs...they died.   Added some koi again....they died.   It seems that koi, commets, goldfish tinfoil and barbs keep dying

Answer
I'm sorry about all the dead fish.  I suggest having your water (both the tap water and the water in the pond) tested for these things or as many as you can - pH, hardness, alkalinity, ammonia, nitrite, nitrate, phosphate, copper, iron, oxygen, chlorine, salt, pesticides, etc.  There are companies that test water but most of those have test kits for them.  There may be something about your water chemistry that is okay for some fish but not for others.

How many 16" koi did you have in the pond during that first die off?  If it was more than a few, then I would suspect that there were water quality issues due to overcrowding such as high ammonia or nitrite or low oxygen.  Koi really need a pond over 1000 gallons and then 100 gallons for each additional koi over that first one.   Koi grow large, sometimes to 3 feet long.  Adult female koi will always have eggs at some degree of development inside their bodies so it's not a surprise that you found eggs in the females.  If a female became egg bound and couldn't lay her eggs, that could lead to internal bacterial infections.  If you had male koi, that's not likely.  I suspect that was a water quality problem.

Why you have since lost fish may be due to something intrinsically off in your tap water.  Do you add dechlorinator with new water to the pond?  Do you do water changes?  Do you add any pond salt?  I suggest 0.05% pond salt.  Do you ever do water changes?

The larger fish that you've had die require higher oxygen levels then the smaller fish.  Plecos can gulp air and live in lower oxygen waters.  What kind of aeration and filtration do you have?  It may not be adequate.  

I hope you can figure out what is going on.  In the mean time, I suggest sticking with the fish that you have who are doing ok and not adding more fish.  You are lucky living in a warm climate that you can keep tropical fish in your pond.

Good luck!