Koi, goldfish and other finned friends are some of the easiest pets you’ll ever own. Watching them flit around the water is one of the many benefits of pond ownership. But you can have too much of a good thing.
Fish don’t thrive in overcrowded ponds. At best, they’ll feel stressed. At worst, the levels of built-up fish waste and other bad stuff could kill them.
So how many fish should you have in your pond? There are lots of meaningless numbers written about this. But the answer is a little more complicated than a simple number of fish per gallon. It all comes down to how well your pond ecosystem handles the fish waste. The bigger the biofilter, the more fish waste you can process.
If your pond has fantastic filtration, you may be able to get away with more fish. If your filtration is less than perfect, your pond might not be crystal clear, and your fish may experience health issues.
The amount of fish that’s right for your pond depends on the size of your biofiltration and how well the beneficial bacteria can process the ammonia and nitrites.
- Our typical Lilypad Ecosystem Pond is 800 to 1,200 gallons with a 1000 Series Biofilter. It can handle about 12 to 15 mature goldfish.
- The Lotus Ecosystem Pond (1,800-2,500 gallons) with the 2500 Series Biofilter can handle either a mix of 12-15 goldfish plus half a dozen koi, OR one dozen koi.
- Our Oasis series Ecosystem Ponds (4,500 gallons and up) are designed to handle much bigger quantities of mature koi. With the extra depth and huge custom biofilters, they can typically provide a beautiful home to two dozen or more koi and other pond fish.
Check out our Idea & Price Guide for more information about our Ecosystem Ponds.
At a Glance: How Many Fish Can I Have in My Pond?
- Water quality and fish happiness is directly related to how well your system processes fish waste, ammonia and nitrites.
- The quality of a pond’s filtration is the No. 1 factor determining how many fish a pond can safely hold.
- Fish in overcrowded ponds are more susceptible to illness.
- Fish can outgrow the size of the pond. Fish have babies. Always account for fish growth and reproduction when adding new fish.
The Dangers of Overcrowding
Overstocking fish is one of the most common mistakes pond owners make.
Some can’t resist the urge to add fish to their collection, while others don’t account for how big their fish will grow. Sometimes, a pond will start off with a reasonable number of fish but become overcrowded when those fish have babies.
No matter how it happens, overcrowding can create a dangerous situation for your pond residents.
Much of the danger comes from ammonia – a product of fish waste and decomposing organic debris (like uneaten fish food). In a healthy pond ecosystem, beneficial bacteria break down this ammonia, converting it into nitrites and then nitrates. If a pond has too many fish, though, the bacteria can’t keep up.
Built-up ammonia can kill fish – and it’s not the only problem in an overcrowded pond. A large fish load will throw the entire pond ecosystem out-of-whack, affecting everything from oxygen levels to pH.
Some ponds might seem healthy with huge numbers of fish packed in a tiny space. But unless that pond has excellent filtration – and someone dedicated to regularly testing the water – the fish are probably stressed and consequently more likely to to get sick. And when stressed, crowded fish get sick, they’re much less likely to recover.
Fighting Overcrowding with Good Filtration
Filtration is the No. 1 factor determining how many fish your pond can hold. Good filtration is the reason stores can safely house dozens of fish in a small tank. Bad filtration, on the other hand, can make even a good-sized pond uninhabitable for pond life.
But what is filtration – and how do you know if yours is good enough for your fish population?
We’ve learned over the past 30 years that pond ecosystems thrive when they have two types of filtration: skimming and biological.
Biofiltration is the word we use to describe the natural processes that keep your water crystal clear and your fish happy and healthy.
Here’s how it works: beneficial bacteria colonize on all the nooks and crannies in your pond. Some of these bacteria consume the ammonia produced by decaying organic material and convert it into nitrites.
If the cycle stops here, these nitrites become food for the kind of single-cell algae that turns your water green. With enough help, though, other bacteria convert these nitrites into nitrates, which become food for pond plants.
The best way to encourage beneficial bacteria growth in your pond is to install a biofilter.
The biofilter typically sits at the opposite end of the pond from your skimmer. Here’s how it works:
- Water flows into the swirl chamber at the bottom of the biofilter.
- Beneficial bacteria grow on the media inside the biofilter – typically filter mats and BioBalls or lava rock.
- These bacteria convert ammonia in the water to nitrites, and the nitrites into nitrates.
- Water flows out of the biofilter through the waterfall.
- Plants – in the biofilter’s built-in plant shelf and in the rest of the pond – consume the nitrates.
This ammonia-nitrite-nitrate cycle is crucial to your finned friends’ health. If the water doesn’t have enough bacteria to complete the cycle, you’ll quickly run into issues like green water, excess ammonia and sick fish.
Make sure your pond has a biofilter big enough to handle the number of fish in your pond. You should also add regular doses of additional beneficial bacteria via a pump bottle or automatic dosing system.
Note: If you have a large pond, your biofiltration might look a little different. Check out our post on Swim Ponds to learn more.
A pond skimmer serves two purposes: it houses the pond pump, and it removes 80 to 90 percent of the debris that enters the pond, including leaves, twigs, pollen and fish waste.
Without a skimmer, all of this waste sinks to the bottom of the pond and decays, causing ammonia and other noxious compounds to build up in the water. That means a pond with inadequate skimming filtration will be able to hold significantly fewer fish.
If your pond has a rubber liner and no skimmer, you can install one relatively easily. But if you can’t – or choose not to – install a skimmer, you’ll need to do the work of pulling all that debris out of the pond yourself the old-fashioned way: a net, a bucket and a pair of sturdy gloves.
How to Tell if You Have Too Many Fish
1. Determine the Size of Your Pond
The quality of your filtration is the No. 1 factor determining how many fish you can have in your pond. Still, it’s helpful to have a rule of thumb with which to start.
Remember: Some recommend housing no more than 1 inch of fish for every 10 gallons of pond volume. If you are unsure of your pond’s potential, limiting the population to 1/2″ per 10 gallons is even better – and gives you some wiggle room when those fish grow and have babies.
Calculate your pond’s approximate volume with the following formula: Length (ft.) x Width (ft.) x Average Depth (ft.) x 7.48 (or use our online calculator). Ponds often have irregular shapes and varying depths, so just take your best guess with the measurements.
2. Assess Fish Health
Stressed, overcrowded fish are more prone to illness than fish that have plenty of space.
Signs of illness in fish include sores, fungal growths and parasites. Some diseases may alter a fish’s behavior, causing it to swim oddly, scrape against rocks or isolate itself from pondmates. Fish starving for oxygen will often gasp at the water’s surface, while underfed fish might have thin bodies and bulging head.
In short, anything that seems “off” about your fish’s appearance or behavior could be a sign of illness. Visit or give us a call at Splash to determine the best course of treatment – then take a hard look at the number of fish in your pond.
If you find yourself dealing with a nonstop barrage of sick fish, or they don’t respond to treatment, you might have too many fish.
3. Assess Water Quality
There’s no need to test your water if everyone in the pond seems healthy. If something seems off, though, it never hurts to check.
Bring a small water sample to Splash for free testing, or buy a testing kit online. We typically check the water’s pH, ammonia, nitrite and nitrate levels. If the levels are less than ideal, overcrowding might be the cause.
By the way – green water isn’t necessarily unhealthy. Humans often don’t like the look of it, but it won’t hurt your fish.
More Tips for a Healthy Pond
The healthier your pond, the more fish you can safely hold.
Here are some more tips for keeping your finned friends happy and healthy:
Add regular doses of beneficial bacteria
Add a healthy dose at least once a week during the warm months, or more often if you feel your pond needs it. Beneficial bacteria is completely safe for fish and plants.
You can also use an automatic dosing system to do the work for you.
Run an aerator to boost oxygen levels
This is especially important if the pump isn’t running (i.e. over the winter) and on hot days.
Don’t overfeed your fish
Only feed fish once or twice a day during the season, and only give them as much food as they’ll finish within about 5 minutes. Don’t feed fish when the water temperature is consistently below 55 degrees.
Use Sludge & Filter Cleaner
This anaerobic bacteria is made specifically to eat away muck on the bottom of your pond and on your filter mats.
Use it in addition to regular beneficial bacteria.
Keep a bottle of Pond Detoxifier on-hand
Pond Detox removes harmful chlorine and chloramines that are often found in municipal water sources. It’s good to have around if you accidentally leave the hose running when topping off the water level in the pond, or if you need to do a water change.
We recommend using double the dosage listed on the bottle to be safe.