I am having the temp problem too and mine is 750 gallons. All of my goldfish have died and my temp is ranging from 80-90 degrees. We should have built our pond deeper, it is around 1.5 to 2ft deep. With the temp highs here being in the late 90's, there is not much to do other than those coolaroo sails...:0(

To the OP. A 100 gal tank is WAY to small for a Koi. It will outgrow it within a year.

Also, that's really too small of a pond for regulating the temp very well (as you are finding out).

I'd stick with plants and maybe a goldfish.

Is your pond in-ground? I agree about 100 gallons being too small for koi.

I wouldn't even put one goldfish in a pond that small. As you already know, it is too hard to regulate the temperature.

DH says if your pump is heating up that much that it will burn out soon. They aren't supposed to run that hot. Something is wrong.

We've had a pump buried in a basket of rocks for years but we used lava rocks because they don't fit tight together like pea gravel does. Better to build a pre-filter out of something less heavy though like nylon scrubbies. Less work.

Thanks everyone. I put in a smaller pump and ran some fresh water in. I'll see later if it stays cooled down. I live in the Nortmost part of Wisconsin so it isn't normally very warm here. The koi i am getting are babies. The guy who is giving them to me will take them back before winter. They are less than 6 inches, and i plan on getting 3 fish.

My pond is inground. It is possible that it is 200 gallons. I am guessing. The water level is 1 1/2 ft deep, 4 ft long and 3 ft. wide. It is in the shade most of the day, because i planted a shade tree next to it, and i live in the woods. I need the fish to eat the mosquitoes! We don't have any stores here that sell goldfish, or any kind of fish.

Do you still think Koi are a bad idea? I have three plants that are growing well.

Yes, your pond is too small for koi even at 200 gallons. Three six inch koi need more like 1000 gallons and they don't stay that small for long.

You can use Mosquito Dunks to control mosquitoes. They are safe for pets, kids and wildlife. Available at Home Depot or online.

Here is a link that might be useful: Mosquito Dunks at a very good price

Wow what huge pools! I got a small 1.5ft wide x 2 ft long x 1 ft deep preform in my front yard in full sun. I have poured in river sand and planted Sagittaria subulata and have water hyacinth and parrots feather on the surface. I originally wanted it for the use of birds to drink from. I put some minnows I bought at the local Petsmart into it and they are still alive after some 2 years. Because it loses water through evaporation and I believe a very slow leak I top it off daily with tap water.
It works and the fish and plants live. What more could you ask for?

FYI

I agree with terrestrial man
I had a 5x6 by about 1.5 ft deep had 8 koi in it for 5 years
before I built a 3000 g pond had no problems with the fish
my biggest one grew to 18" The KEY is to have good water flow keeps the water cooler.
Yes I had to fill my pond up every couple of days but I had
a really good bio filter & a small skimmer had sun part of the day shade the rest of the day. But now I am glad I built
a bigger pond & love it.

Well the smaller pump allowed the water to cool so much that i had to put the heater in! The koi were only 2 1/2 inches. I put them in two days ago. I like the idea of having a huge pond, I just can't do that right now. I had three large koi in my 400 gallon pond for about 5 years. Never had one even try to jump out. They were healthy and happy. My Butterfly was about a foot long. She was the biggest fish i had. Minnows is a good idea!

Are you protecting them somehow? If you live in the woods, you're going to lose them if you don't somehow protect them.
There's cats, coons, heron, etc., etc.
Good luck!

Just because you can keep a koi alive in a tiny pond doesn't mean they are healthy or growing properly. A five year old koi that is only a foot long is not of normal size. Koi raised in ponds that aren't very deep don't develop correct muscles either because they need to swim vertically as well as horizontally to do so. 24" is a normal size for koi to reach and they can live for many, many years.

I fail to understand why people insist on koi when goldfish are so much more suited to their ponds. There is nothing at all wrong with having them instead and nothing at all wrong or less with having a smaller pond. One isn't less of a ponder or fishkeeper, koi aren't the ultimate prize. It isn't a criticism or a negative advising that a pond is too small for koi. You can take a Great Dane or German Shepherd out of a tiny apartment for daily walks but the fish are stuck.....