Before I came to Play It Koi, I spent 16 years as the general manager of a big box pet store with an outstanding aquatics department, and I ran thousands of water tests for customers there. Most of those tests were for aquariums, and over those years I helped a lot of people work out what was going on in their tanks. Most of that carries straight over to ponds. In most cases a koi pond is an outdoor aquarium on a bigger scale, and most of the water chemistry works the same way.
One problem I dealt with when helping customers at the store is what aquarium keepers call old tank syndrome. It happens in ponds too, and in a pond I call it old pond syndrome.
What is old tank syndrome?
Old tank syndrome is a slow chemical drift that happens when a tank runs for a long time without enough real water changes for the number of fish living in it. Nothing breaks and nothing looks wrong. The water keeps changing, a little at a time, for months or years, and the fish adjust as it goes.
While that's happening, waste builds up. Your filter turns ammonia into nitrate, and plants and algae only use up part of it. The rest stays in the water until you take it out with a water change. So do phosphate and the rest of the dissolved waste. Topping off the water that evaporates makes this worse, because evaporation only takes out pure water. Everything dissolved in it stays behind and gets more concentrated.
At the same time, your buffer runs out. Your filter bacteria make acid while they convert ammonia, and your KH neutralizes that acid. KH is your water's carbonate alkalinity, the reserve that keeps your pH from moving, and every day some of it gets used up. When nothing replaces it, KH eventually hits zero and the pH starts sliding down, often to 6 or lower. I go through how that works in detail on our KH page.
The fish that kept dying
One customer at the store bought new fish from us week after week. Within a week they'd be dead, so he'd come back and buy replacements. After weeks of this, I finally talked him into bringing in a fresh water sample.
When I tested it, his nitrates were through the roof, past the top of the range our test kit could read. He was confused. The fish that had been in his tank for years were doing fine, so why did the new ones keep dying?
I explained that the fish he already had had been getting used to those conditions slowly, over a long time. The new fish never got that time. I also told him that for the long-term health of the fish he had, it would be a good idea to start correcting it.
He told me he was already doing a water change every week. Only after we talked for a while, and I had him walk me through exactly what he did, did the real story come out. What he called a water change was topping the tank back up after a week of evaporation. No water was ever coming out.
I gave him a plan: very small partial water changes, and adjusting the water chemistry over time.
The next week he came back. He had done a 90% water change and swapped out all of his filter media at once, and every fish left in the tank had died.
Which brings me to something I tell people all the time: nothing good happens quickly in a pond or an aquarium. Sure, there are exceptions to the rule, but in most cases slow and steady wins the race.
How old tank syndrome turns into old pond syndrome
Does a pond really get the same thing? It sure does. Some things about a pond slow it down, and some speed it up.
Most ponds hold a lot more water per fish than an aquarium does, so the drift takes longer. Plants and algae take up some of the nitrate, and rain overflow swaps out a little water now and then. Heavy rain also dilutes your KH along with everything else, so after a wet stretch it's worth retesting.
The auto-fill valve is the one to think about. If the only water your pond ever loses is evaporation, the auto-fill is doing the same top-off routine my customer had, just automated, and the waste is concentrating. If your filter flushes to waste, or you drain water off when you clean, the auto-fill is replacing that water too, and you're getting some real water changes without thinking about it. It helps to know which one your pond is doing. And if your source is city water, remember the auto-fill brings in chlorine or chloramine along with it.
Leaves, wood and sludge break down and release tannins and other dissolved organics, which tint the water and push pH down a little more. And koi grow. A pond that was lightly stocked five years ago may not be today, and everything big koi eat ends up in the water one way or another.
So old pond syndrome usually shows up in ponds that have been running for years, where the fish look fine and the owner hasn't changed anything in a long time. Nothing seems wrong, so nobody goes looking.
Signs of old pond syndrome
If I see these in a pond, I want to rule it out:
- New fish die within days or weeks of going in, while the fish that have been there for years seem fine.
- Fish die right after a large water change.
- Your pH reads low or keeps creeping down, and your KH is near zero.
- Your nitrate is high. On the API nitrate test, I like to see it under about 20 ppm, and over 50 ppm means waste is building up.
- The water has a yellow or tea color even though it's clear. Pour some into a glass and hold it against something white. Color stacks with depth, so a tint you can barely see in a glass can hide the bottom of the pond.
- Plants are growing like crazy. That isn't a problem by itself, but the vigor tells you the nutrient load in the water is high.
- Foam sits on the surface and doesn't break up, outside of spawning season.
- A thick layer of sludge has built up on the bottom.
- The fish are sluggish, slow growing, or losing color, and algae is a constant fight.
None of these alone proves anything. Several of them together, in a pond that has been running for a long time, is what makes me suspicious.
Why a big water change can kill fish in an old pond
Your test kit's ammonia reading is total ammonia, and it's made of two forms. Ammonium is far less harmful. Free ammonia is the toxic one. How much of the total is free ammonia depends mostly on pH, and warmer water pushes it up too. At pH 6 it's a tiny fraction of a percent. For each full point the pH goes up, that share goes up roughly ten times, so by pH 8 it's around 5%. That still sounds small, but it doesn't take much free ammonia to hurt fish.
So picture an old pond with a pH around 6 and some ammonia showing on the test. The low pH has been protecting the fish. Then the owner does a big water change with tap water at a pH of 7.5 or 8. With no KH left to hold it, the pond's pH jumps toward the tap water's in an afternoon. The water change takes some ammonia out, but the toxic share of what's left can go up 50 to 100 times, so the fish end up worse off.
That isn't the only thing that can go wrong with that much new water at once. Untreated chlorine or chloramine, a big temperature difference, or excess dissolved gas in fresh tap or well water can each kill fish on their own. And fish that spent years adjusting to the old water get thrown into something completely different all at once.
Swapping out all the filter media at the same time takes out the bacteria that were processing the ammonia in the first place. My customer did a huge water change and replaced all his media in the same week.
How to test for it
To spot old pond syndrome you need the two numbers most people never test: nitrate and KH.
The API Pondcare Master Liquid Test Kit is a good kit, but it tests pH, ammonia, nitrite and phosphate. It doesn't include nitrate or KH. If you'd rather have everything in one box, the NT Labs PondLab 200 (NT280) is the kit I point people to. It tests ammonia, nitrite, nitrate, pH, GH and KH, so nothing gets skipped because it wasn't in the kit. One thing to know: NT Labs reads nitrate as nitrogen, so the nitrate levels I give below come out about 4.4 times smaller on its chart.
If you already have the API kit, add the API Nitrate Test Kit and the API KH Carbonate Hardness Test Kit. The API KH test counts drops, and each drop is about 17.9 ppm, so the 100 to 150 ppm range I recommend works out to roughly 6 to 8 drops. All of our pond test kits are on one page if you want to compare.
Knowing how your source water tests matters just as much as knowing how your pond tests. Every drop of water you add, from a hose, an auto-fill or a water change, pulls your pond toward whatever is in it. Test pH, KH and nitrate on your tap or well water at least once, and again before you start any correction. If your source water is soft, water changes will lower your KH instead of raising it. If it has nitrate in it, water changes can only bring the pond down to that level. City water can also shift with the seasons. Usually it isn't by much, but it does happen.
On city water, check your utility's water quality report to see whether it uses chlorine or chloramine. Spraying the hose to let it gas off works for chlorine, but it doesn't work for chloramine. Letting chloramine spread through the pond only dilutes it. It doesn't destroy any of it. Dose your conditioner for the amount of water you're adding, put it in first where the circulation is strongest, and then start filling.
Whatever you test, write down the actual numbers. "Tests fine" doesn't tell you or anyone helping you very much, and the trend from one test to the next is often what gives the problem away.
Test pH early in the morning and again in the late afternoon. Ponds differ from aquariums on this one. Algae and plants pull CO2 out of the water in daylight and push pH up, and when your KH is low, that daily swing can be a point or more. A single afternoon reading can look fine and hide the problem.
The classic pattern looks like this:
- pH low, often 6 or below
- KH near zero
- nitrate high
- nitrite often at zero while ammonia slowly climbs, because the filter bacteria slow down at low pH
If your auto-fill brings in hard water, it keeps putting KH back. A pond like that can show high nitrate with normal pH and KH. That's still old pond syndrome. It's just missing the pH crash.
How to fix old pond syndrome without losing fish
Walk the chemistry back the same way it got there: slowly.
- Test everything first. Get pH, KH, ammonia, nitrite and nitrate from the pond, and pH, KH and nitrate from your source water, so you know where you're starting and what each water change will actually do.
- Stop feeding while ammonia is showing. Less food means less ammonia while you work on it. Once ammonia reads zero, start back slowly. Our feeding guide by water temperature can help you figure out what your fish actually need.
- Keep the aeration running strong. Your fish and your filter bacteria both need oxygen while things shift. If yours is weak, look at our pond aeration options.
- Get the ammonia down before you raise the pH. Every water change in a pond with no KH raises the pH, because your tap water carries KH with it. Use a water conditioner that also detoxifies ammonia, not only one that removes chlorine and chloramine.
- Make small water changes, about 10% at a time, a couple of days apart. Match the temperature of the new water to the pond, and treat every bit of it with conditioner. Check pH a few hours after each change. If it jumped more than about 0.2 to 0.3, make the next change smaller.
- Bring KH up in stages, after ammonia is under control. Raise it a little at a time and retest in between, using the measured baking soda doses on the KH page. Don't put in a full label dose of a buffer all at once in a pond that has crashed.
- Clean out sludge in steps. Stirring up years of muck all at once can release ammonia and use up oxygen. Think of it less as vacuuming and more as a series of targeted partial water changes. The water a pond vacuum pulls out goes to waste, not back into the pond, and it counts toward your water change. Take out the worst of it, wait, then do more. Keep air stones out of black sludge too. That sludge has no oxygen in it, and blowing air into it can release hydrogen sulfide. The tell is a rotten egg smell.
- Leave the filter media alone as much as you can. If it needs cleaning, rinse it in a bucket of pond water, not tap water. If you have more than one filter, don't clean them on the same day, and never replace all of the media at once.
After any big disturbance, like a sludge cleanout or a filter cleaning, give the pond about a week before you do anything else. More intervention that week makes it worse, not better.
Bottled bacteria won't fix old pond syndrome. Bacteria are the right tool for maintaining a pond that's already clean. Tea-colored water from tannins only clears with water changes or activated carbon.
Retest after each step, and keep testing nitrite through the recovery. As the pH comes back up, the bacteria that eat ammonia wake up faster than the ones that eat nitrite, so nitrite can spike for a while even though it read zero before. If the numbers move the wrong way, stop and figure out why before you do anything else.
How long does it take? Let your test results set the pace, not the calendar. Each 10% change only takes out about a tenth of the nitrate, so getting very high nitrate down to a healthy level can take fifteen or more of them. That can mean a month or more. It took months or years to get there, so a month is a good trade.
How to prevent it
Do real water changes, where water comes out and new water goes in. Topping off doesn't count, so if your auto-fill only ever replaces evaporation, you still need to drain some water out.
Test nitrate and KH, not only pH and ammonia. During the feeding season, test KH weekly and nitrate at least once a month. For most koi ponds, holding KH at 100 to 150 ppm gives you a comfortable reserve, and a buffer like AquaMeds Buff-It-Up makes routine top-ups easy once the pond is stable.
Stay on top of sludge and leaves before they pile up, and feed for the water temperature, not for how hungry the fish look.
Don't add new fish to a pond that's in the middle of a correction. Wait until the numbers are back where they should be. When you do add fish, quarantine them and bring them over slowly. If your pond water is very different from the water they came from, take more time.
Questions I hear about old tank syndrome
Can old tank syndrome happen in a koi pond?
Yes. The chemistry is the same. A pond has more water per fish, so it usually takes longer, but an auto-fill that only tops off, organic buildup and growing koi can make up the difference.
Is old tank syndrome the same as new tank syndrome?
No. New tank syndrome is a filter that hasn't built up its bacteria yet, so ammonia and nitrite spike. Old tank syndrome comes after years of slow buildup in a system that has been running fine.
Does topping off count as a water change?
No. Evaporation takes out pure water and leaves everything else behind. A water change means removing old water and replacing it.
Why are my pond fish dying all of a sudden after a water change?
There are a few common causes: untreated chlorine or chloramine, a big temperature difference, or a fast pH jump. If the pond has been running a long time without real water changes, that pH jump can turn ammonium into toxic free ammonia. Test pH, KH, ammonia and nitrate, and keep the next changes small.
Need a second set of eyes?
If you think your pond might have old pond syndrome, or you've got test results you're not sure about, we're happy to take a look. Call or text us at 206-350-7580, or email [email protected]. Send your pond readings and your source water readings together, so we can see both sides.