I agree, these fish could well have been ill when you bought them. The same thing has happened to me recently when I tried to introduce some minnows (striped shiners) into my 55g indoor aquarium. Deaths at a high rate, signs of distress, and ultimately, all died within two weeks. In the end they were patchy, discolored and coated in fungus. :( You don't say much about your pond set up, or how you introduced the fish. I know my biggest problem when I put the minnows in my tank, is that I did not account for temperature needs. The minnows, being cold natured fish, could not survive in my tank seeing as the water was warmer than they were used to and therefore less oxygenated. Even with an air stone and filter, they could not get enough oxygen. The stress from moving only magnified their existing problems - which were plenty, seeing that they were bait fish and were not babied as most pet fish are. Add to that fact that my water conditions were less than ideal for them, their death was pretty much assured. In the future, I will do more careful research about the fish I introduce to be more certain of their needs (rather than assume), and only choose healthy specimens from a reliable source. Plus, checking and rechecking my water parameters before putting them in, and introducing them in the gentlest way to minimize the stress. Sorry about your fish, good luck in the future though!

First off, you don't need to empty and refill a pond due to algae. Nothing is fixed, you just started the cycle over. I don't have visual, but I'm guessing a lot of the stuff you scrubbed was actually beneficial carpet algae. Some is string algae if it gets very long, but all that is actually healthy stuff. Water treatment-wise...you need to know whether you use chlorine or chloramines in your area. Most treatments take care of both, but not all. Algae fix is a solid product, but in extreme heat, especially in above ground pond, even a 'proper' dosage can kill fish. Rare, but if very hot it takes out the oxygen from the water and your fish can die. With all of that said, it was probably ph or temperature shock that killed them. Could have been one of the treatments, but less likely. Future reference, never drain and refill a pond unless directed by a professional. About the only time that is a good idea is an extensive infection or infestation of some type. You just start the algae cycle over, nothing is fixed. Be patient and treat the water, than add cheap fish. If they survive than you can add more or better. Stay with goldfish, koi need a lot more water than what you say you have. Good Luck.

How quickly did they die? Did it take a couple hours....a couple days? Overnight? Possibility 1: Did it rain the day they died? Check your pH and totak alkalinity (kH). You may have had a pH crash. Possibility 2: do you have rocks in the bottom of the pond? Have they ever been removed and the gunk between them cleaned out? Are there any unused pipes running into the pond? Are your filters clean or is there gunk sitting in them? Hydrogen sulphide gas may have killed them. Hydrogen sulphide gas forms in decaying materials. It smells like rotten eggs. If a gas bubble formed in the pond and released, the entire contents of the pond would die pretty quickly. possibility 3: Have you added any new fish lately? the new fish may have been sick with parasites or other disease.

No to tell without an autopsy. Since that is just not realistic with a $1 fish, I would say that petiolaris is on target. These things happen with aquariums. Such is the life of an aqua hobbyist. For what it's worth, I have a ton of platy's over the years. Some last a long time, some not so much. Only a few times one or two had some sort of symptom for a week or less then died. Most I just found randomly dead without warning.