As suggested in another post, I suspect birds or something else are taking your fish. They can be picked off if too exposed. Or, are you actually seeing dead fish? We have posters with galvanized containers like yours who do not have issues. I guess you could have a poorly made one though.

Unfortunately we see the dead fish...

Put a liner in it is about all you can do really. You know people say all kinds of stuff on the internet - as with anything else read from a variety of sources & usually you start to get some idea of whats most likely to be true. It seems to be pretty commonly understood, even amongst fish farmers and the like that galvanized tanks = zinc toxicity. Who knows why some people might say they have fish that are alive in a galvanized tank - maybe there is lime deposit coating the tank or an old tank that the zinc has mostly leached out.. Or who knows - maybe after they wrote that they went out and found their fish dead.

The fact that they're dying so swiftly does seem like some sort of toxicity - are you dechlorinating the water? Not overstocking the pond? (Ammonia/nitrite can build up prettyquickly and overwhelm the nitrification cycle if too many fish and/or organic matter and not enough filtration.)

Another possibility is excessive heat. Since your pond is completely above ground, there is no insulation from sun as would exist if the pond were surrounded by soil. As with most posts on gardenweb, your location or zone is useful information.

Thanks for everyone's input. I am west of Chattanooga TN in a valley so Zone 7A. This year the fish died earlier in the summer/late spring so temps were not excessive. The pond starts having shade about 4:00 in the afternoon. Have definitely not had too many fish, but I do keep plants as seen in the photo. Would fertilizing the plants with plant food tablets for ponds be too much? Have use dechlorinator, water conditioners, ph down at various times. PH is on the high range of the scale and doesn't usually change with treatment. Last fish die off was after using the water conditioner which says it will remove zinc. Trough was a new purchase from Tractor Supply 2 summers ago. We are thinking of using a pond liner, but not sure how we would hold in place at the top. Any ideas or more comments?

Doing a complete water test wouldnt be a bad idea- either get a test kit or an aquarium specialty shop can do it. (the little test strips they use at petsmart may not be accurate). While not as immediately lethal as ammonia/nitrite, it is possible to have too much nitrate.

First thing I'd say is stop using all those treatments and dechlorinators. There are so many times I hear of fish dying and it was right after using all that junk. Sure you don't want to plop a fish in a brand new tank just filled with a hose. But an established tank I've never ever once had to use anything when doing water changes. Especially with goldfish. Just let the water do what it's going to do for now. You can cause all kinds of crashes and up and downs constantly if you keep messing with it. Let it find a balance itself. Is that your actual pond? The water looks very clear for having no filter, all plants in a pot, and in the sun. Are you using algaecide or something to clear it? Stop using that too if you are.

A stock tank shouldn't ever be a problem either. I've seen plenty of people use those and never have dead fish. Something else is going on. I'm assuming you are doing normal water changes. Which shouldn't allow any metal to build up anyway. And hopefully you don't have tons of rotting plants and leaves on the bottom. If you do that could be a problem.

You should really get some type of biofllter or something though if you are going to keep fish in it. But still just a few fish in that large tank shouldn't even be tough to keep alive as it is. At the very least run the water through some of the plants roots. The plants should totally take care of anything bad in the water. But the roots need to get to it. Try getting some floating plants like water hyacinth and run the water through those roots, those are super filters. That should clean anything right up out of there. If you can get some yellow flag iris and leave them bareroot with water flowing through the roots they will suck every last bad thing in the water and purify the heck out of it. I see plenty of plants that would work great actually if the roots were out in the water.

So I'd say do a 50 percent water change, then don't touch it for a week or two. Let it get established and balanced, don't add anything to the water or mess with it. Any chlorine will dissipate quickly by it's self before you put fish in again. At the very least get some water hyacinth and run the water through it. Then introduce some new fish. I'm betting you won't have a problem after that.

For what it's worth I have a stock tank also about the same size. But mine is plastic, however I have two metal stock tanks that the water also goes through. A 4' round and 3 foot oval metal ones I use for filtration. Have about 3 dozen goldfish, some over a foot long. And they keep having babies. Only lost one in almost 4 years and that was from a massive heatwave. Never once used a single product in the water, hose water never bothers them when doing water changes of reasonable size and not an overwhelming amount. People always freak out about hose water, but a 20 percent change diluted into the rest of all that pond water isn't going to do diddly to anything. Shoot my fish come swim all over into the hose water and play, they love the fresh cool water.

Thanks T. I'm thinking like you. I'm going to stop using the chemicals. Haven't used anything in a couple months and the water is looking good. That is the closest thing I've come to that seems to correlate to the timing of the fish dying. It may be that I am causing ups and downs that kill the fish. Yes that's my pond and we have a biofilter in the pond. We did a total water change this past spring when I repotted and fertilized the plants and the water got very cloudy and wouldn't clear up. Tried using algaecides at that time but haven't again in months. That was another problem time. I used oil dry as the medium for the plants. Have read that is fine and cheaper than what you buy at a water garden center. Any comments? We had no fish at that time but added some several weeks later. They also died. Believe I did use the water conditioners after adding the fish to remove the zinc. I have 3 water lettuce as the floater plant and will add more. Would like to add the yellow flag iris. Are you saying don't put them in a pot??? What do you mean use bareroot in the pond? Are you saying do that with the other plants I have?

It's been a couple months since the last fish died or chemicals were used. So I think the water should be pretty settled by now. I'm thinking about going out this weekend and getting some goldfish and trying again.

Oh yes, cool set-up you have with your ponds and the filtration levels!

One other thing, regarding water changes. I don't do more than top it off unless I have a major problem like described above with the cloudy water.

If you have hard water galvanized can be fairly safe but if you have soft water the zinc will destroy the chloride cells in the gills which normally take up Calcium or magnesium which are lacking in the soft water.

What soil are you using in your plant pots? You should use straight 100 % clay kitty litter with no additives, dont use Miracle Grow or other potting/garden soils with slow release fertilizers in it. The fish will fertilize the plants just fine.

I never do more than a 50% water change, 30% is better.

Maybe put in a fish liner as you first thought.