Before you get all excited and start planning on which veggies you intend to plant in your vegetable filter I better tell you that a “vegetable filter” is a term used to describe a type of filtration where the water passes through a bog pond. The use of a vegetable filter as your only type of biofiltration can be deadly unless you know the whole story. It’s true that plants do a lot of good but that’s only part of it.

First of all you must understand that everything in a pond is based on a unique situation. No two ponds are alike. Everything pertaining to a particular pond affects another aspect. For example, a pond that is in sun and has no fish will grow beautiful waterlilies, requires basically no other form of filtration, and is subject to extreme algae blooms because of the amount of sunshine but miraculously may easily balance and turn clear because there is no fish waste as added fertilizer. Take that same pond and add only a few fish. This may throw off the balance and the water may never clear up because fish waste contributes to the amount of fertilizer in the water making it more difficult for the plants in the pond to absorb all the nutrients and starve out the algae. Without added biofiltration the fish are apt to get sick because there’s not enough nitrifying bacteria to develop the nitrifying cycle that must be in place to change deadly ammonia and nitrites into healthier nitrates.

Plants and Free-Floating Algae

Plants absorb fertilizer (nitrates) in the water and can starve out floating microscopic algae. This helps with clearing up green water but doesn’t do much for string algae.

Plants and Dissolved Oxygen

Plants create oxygen in the water on sunny days due to photosynthesis. At night, however, it’s the opposite. They actually steal oxygen from the water. A night following a dreary day can spell trouble for fish that depend on dissolved oxygen in the water to breathe. Little oxygen is generated by the plants during cloudy days because photosynthesis is minimal. Plants then take what’s left of the dissolved oxygen during the night. Many people make the mistake of turning off the oxygen supply (the water fountain or waterfall) at night and can’t understand what happened when they find most of their fish dead in the morning.

Another way plants affect the amount of dissolved oxygen in the water is when they die. The process of decay uses oxygen. If you were to kill all the algae in your pond with one blow, perhaps by using an algaecide or herbicide, the pond would suffer an extreme loss of dissolved oxygen and literally smother your fish.

Plants and pH

The process of photosynthesis also affects pH. In the morning after the pond is depleted of dissolved oxygen the pH is at its lowest. It will be opposite in the evening. The pH will be higher during the evening hours because plants have been taking the CO2 out to the water and giving off oxygen. This swing in pH is more pronounced when there is a lot of algae (or plant life) in the pond. A change of pH more than a couple of points can kill fish so it’s important that the water is buffered against wild fluctuations in pH. We recommend using pH stabilizer.

Plants and Shade

Floating plants such as water lilies and water hyacinths cover the pond’s surface and offer shade for fish and protection from predators. Predators cannot see fish when they are under the pads. The pads also offer shade from the blistering sun. (Yes, fish can get sunburned!) This shading of the water also helps with getting rid of algae bloom. Algae bloom requires sun to flourish.

It’s my own hypothesis that algae bloom in the early part of the season has a purpose. I wonder if Mother Nature intended the water to be so green that predators can’t see through it as protection for the fish. When plants get large enough to cover the surface of the pond and offer shelter to vulnerable fish the algae bloom tends to mysteriously go away.

Plants and the Nitrifying Cycle

Plants are part of the cycle that converts ammonia to nitrates. Dying and decaying plants produce ammonia just like fish do when they respire, poop, or die and decay in the water. The other major factor to which they contribute is to remove nitrates (fertilizer) from the water. Reduced nitrates create a less toxic environment for the fish.

Plants and Biofiltration

The use of plants as your only form of bio-filtration can result in fish disease, deaths and an unhealthy environment yet many books and articles written by very famous people in the world of watergardening will condone the practice. I’ve noticed that they often say that you may set up a “vegetable filter” that will keep your pond clear and pretty. Never have I seen where they actually give you the complete story.

They tell you a vegetable filter is copied from Mother Nature and is the “natural” way to filtrate your pond. They leave out the fact that Mother Nature does not recycle her waters with a pump. Her “natural” processes are much more complicated and water passes through many natural filters before it becomes clean water again.

The introduction of fish into a pond adds a new set of rules. A vegetable filter may have worked well for you in the past, providing nice clear water and a great place for frogs to congregate. It will not, however, be enough when you put fish into the picture. Most folks, including us, overstock our ponds. It’s surprising to find that even 3 fish can be too many for many small ponds. As with aquariums, ponds need added filtration to house and promote the growth of the two essential nitrifying bacteria that keep the nitrifying cycle working properly.

For the benefit of those who say their pond has 50 fish and no filtration other than “natural” like a vegetable filter I say, “It’s only a matter of time”. It’s not normal for a few fish to die or get illnesses such as bacterial gill disease. Overcrowded conditions and poor filtration are the culprits.

A vegetable filter, whether it’s used alone or with other methods of biofiltration, must be easy to keep clean. Many of them are built so that water runs over plants in a stream or bed of rocks. This works extremely well to help clear the water because dirt and sediment will fall into the rocks. If you can’t clean the bed, however, it will become a haven for parasites and bad bacteria.

What it all boils down to is that vegetable filters are great if used correctly and not used as a substitute for biofiltration. Water plants perform many beneficial natural functions and beautifully soften the appearance of the pond. A water garden without fish is a perfect candidate for vegetable filtration alone. Add fish, however, and you will eventually have problems if you depend entirely on a vegetable filter.