Last summer, my Koi finally reached sexual maturity - and spawned for the first time! 

YIKES!  Imagine a 50,000 gallon pond with 20 LARGE Koi - and I estimated that they produced over 4 MILLION eggs!  While I know that the Koi feast on the eggs, I also figured that there was no way they could ever eat all of them from that huge pond in the few days before they hatched (generally 3-5 days, depending on water temp).  I have nothing against cute baby Koi!  But even if 1% of the eggs hatched, that meant that my filtration system had to adjust from 20 Koi to 40,000 Koi overnight - NOT good...  So - how to get rid of the eggs?  I particularly didn't want to do anything that might hurt the already-stressed adult Koi, and I didn't want to kill off the filters.

So I did a bunch of research, and talked to some experts.  The eggs are fairly fragile.  The biggest problem facing those that are breeding Koi on purpose is fungus that attacks the eggs.  Fungus doesn't usually attack eggs in well filtered, large Koi ponds, or at least I couldn't depend on fungus to keep all the eggs from hatching.  I did learn that when the eggs are dead, the centers turn white, so that it's easy to tell a dead egg from a live one.

The answer to my problem turned out to be cheap and easy! 

Turns out that a single, 1ppm treatment of Potassium Permanganate (PP) will effectively kill all the eggs!  So I turned off my filtration, mixed up a 1 ppm solution of PP, and distributed it around the pond.  After 2 hours, I poured in some Hydrogen Peroxide as neutralizer (didn't have any Sodium Thiosulfate - another good, inexpensive alternative for neutralizing PP), waited for the water to clear and turned the filtration back on.  The water was polished and beautiful, the Koi were happy and unaffected, and by the next day the eggs turned white.  The Koi were still gorging on them, and there did not seem to be any ill effects from the Koi eating the dead eggs as well as the live ones.

One small problem.  Instead of turning off the filtration, I should have instead just BYPASSED the filters, and let the PP run through the plumbing.  I ended up with 12 eggs that hatched.  I didn't find the fry until they were about 3" - way too large for the adult Koi to consume.  I assume those eggs were hiding in the plumbing and the hatchlings made their way back to the pond.  The pond is so big, and my seine net so large, that I'll have to wait until next year to seine those 12 out of the pond, and donate them to the club as good Koi for newbies to use in their first ponds.  Live and learn!  At least the PP was an easy, successful answer, and next year, I'll be ready for the spawn and know to bypass my filters when using the PP.

Note that PP is fairly dangerous stuff!  Whenever using it - PUT ON EYE PROTECTION FIRST.  Even one drop of PP in your eyes can cause irreparable blindness!  And triple check your calculations for a 1ppm solution!  Remember that PP is an indisciminate oxidizer.  It permanently stains anything it touches - so don't get it on surfaces!  It even takes days to wear off your skin, so gloves are recommended.  Be super careful when mixing and adding to the pond - it splashes!  A 1 ppm dose should be safe for frogs, plants, and other aquatic life, and won't hurt birds that drink the water during the treatment.  That said, try and keep all pets and children far from the pond while treating, even with low doses of PP.