Help! Why Are My Guppies Dying? 30 Gallon Tank

I have a 30 gallon fish pond with 4 Comet Goldfish, 3 Shubunkin Koi, and maybe 50 guppies.
I just moved all of the fish into 5 gallon buckets while I cleaned the pond. I put all of them back in after emptying, scrubbing, and refilling the pond. About 20 minutes later, 17 of the guppies flipped over and died! They were all only about 3 months old.

 

Awe, that’s so sad! Did you add dechlorinator to the water before adding it to the pond?

 

I'm sorry about your fish. That must have been traumatic! Was there a significant difference in temperature between the old and new water? pH? Did you use any chemicals to clean anything? How long were the fish in the buckets? Why did you remove the fish from the pond to clean? It's often easier on the fish to change water and clean things with them in place.
On a separate note it seems like your pond is seriously overstocked. Can you upgrade or re-home some of the fish? If you aren't having water quality issues now you probably will be soon. Goldfish and koi are huge waste producers and need quite a lot of water to keep that diluted. I have two 75 gallon tanks. One has four fancy goldfish and the other has 5. I do huge water changes weekly to keep nitrates in check. They're filthy little water pigs, but I love them!

 

I let the water sit out for 20 minutes or so to let the chlorine evaporate.

 

I don't know anything about ponds, but I do know that with aquariums, scrubbing it clean will get rid of all the good bacteria and destroying the nitrogen cycle. Do you have a pond filter?
It sounds very, very overstocked as well. With so many fish in a small space, and with messy fish like goldfish, maybe ammonia was released in very rapidly in a amount of time?

Also, I don't know if the chlorine evaporated in 20 minutes? Did you test it first?

How to Remove Chlorine From Water

 

The temperature of the old water was about 75 degrees F, the new water WAS 85, but I let it go down to about 80. Is the water supposed to be colder? The pH level was the same, I used a new sponge to scrub the algae and waste off. The bigger fish were in a seperate bucket from the guppies, but they were only in there for around 45 minutes. I thought the absolute minimum for 1 koi was 20 gallons and then one more gallon for each fish?

 

A day is the bare minimum to evaporate chlorine out of the water and 5 degrees is a huge temperature difference for such a tiny fish. They all went to shock. Also some pretty severe stocking issues, too small for even a single koi or comet long term. The (very poor) rule of thumb would be along the lines of 1" of fish per gallon total, and even following that rule you are around 10x overstocked. Koi ponds generally start at a minimum of around 350 gallons.

 

Most likely not using a dechlorinator, temp difference and potential other differences in water chemistry did them in. It does sound like you're overstocked if your pond is only 30 gallons.

 

They stopped dying, but I will try to get a bigger pond. Sorry little fishies.

I just found out that I've been thinking wrong. The pond is actually 45 gallons, but that's still too small right? Even if the fish are only 4 to 5 inches long?

 

I know the post is a month old but my suggestion for the future is to get a clean food grade 55 gallon barrel or trash can. Fill it with water and let it sit 3 days before a water change to be certain. Keep it next to the pond and leave the lid off but cover it with a shady material or cloth to avoid algae. I personally fill my container back up immediately after a water change and if I have to change water mid week I am ready or otherwise I use it for my weekly water change. Also another option for a pond but less pretty is a 275 gallon or 330 gallon plastic tote that's food grade with the top cut out. Makes a great pond if you don't mind the looks.