Agriculture toolbar

Building a fish tank filter

Materials required: An aquarium 10 gallons or more, a submersible pump, 6 foot black plastic hose, plus elbows and fittings, a 10 gallon plastic tub, a ribbed nylon fitting, 5 to 7 gallons of washed pea gravel, 4 to 6 plants, 2 to10 fish. A microbe innoculant for pea gravel such as river mud.

In a biosphere, the wastes for one system are often food for another. Fish in an aquarium eliminate their wastes into their water, and if the water is not changed or purified, eventually it will become too dirty for fish to live. Microbes can break down the fish wastes into necessary nutrients for plants, and the plants then use this food to grow. By using a hydroponic plant grower as a water filter, the fish water can be kept cleaner, longer.

Day 1 - The fish tank and hydroponic should be set up as in the above drawing. Samples of the water to be used for the fish can be checked with available testing kits, and the data should be recorded. pH should be taken with the pH color strips, if they are available.

Hydroponic - The plastic 10 gallon container is used for the hydroponic. A hole must be made towards the bottom of a shorter end. An exacto knife or drill can be used to make the hole. A circle of nylon netting about 4 inches in diameter is cut and placed over the inside hole of the nylon fitting (see Experiment 1). The nylon fitting is then placed in the hole so there is a drain back to the fish tank.

Pea gravel to be used for the hydroponic media should be thoroughly washed and then placed in the tub. Plants are then added to the tub (herbs are recommended). The soil can then be left on the plant roots as they are transplanted as a starter for microbes in the gravel.

The plant hydroponic is a simple tub with pea gravel or grow rock used for media. The plants used in this experiment should be herbs, such as sweet basil, that can stand a lot of ammonia.

A submersible pump is placed in the bottom of the fish tank, and plastic tubing is set up from the pump to the top of the hydroponic. If the hose kinks then elbows and connectors can be used on corners. The pump is turned on, and water from the bottom of the fish tank is pumped to the top of the hydroponic. Water returns to the fish tank through the nylon fitting in the bottom of the hydroponic.

Fish are added to the fish tank. There must be a balance between the number and size of the fish and the number and size of plants. This balance is determined by the experiment. Some plants may be removed if the plants are not getting enough food, and some fish may be removed if all fish wastes are not being used by the plants.

Environmental factors:

The biosphere needs to be in year round temperatures from 50 to 80 degrees F. There must also be enough light for plants to carry on photosynthesis. The plants should receive at least 8 and probably 12 hours of sunlight a day.

Fish Wastes in Water

The fish eat processed food for continuing life and growth, and eliminate 20 to 25% of that food as solid waste. Any uneaten food and solid fish waste is removed by the pump to the gravel bed in the hydroponic. Cleaned water is then returned to the fish tank through the drain hole of the hydroponic.

If there is no place where sunlight is available, artificial light can be supplied with room lights, but this may not be adequate and a small grow light might have to be purchased.

The air and water quality should be good. If water is taken from the city water supply, then it may have residuals of chlorine and chloramines that will have to be removed by a chemical or aeration for several days.

Fish -A fish cannot manufacture its own food from the sun, so it has to get its food from plants or other organisms. The fish has to eat a certain amount of food to maintain its life and to grow. A fish will eat until its nutritional needs and energy needs are met, so a high nutrient high energy food should require less food than a low nutrient value, low energy food.

The fish will eliminate from 20 to 25% of the food eaten in a day as solid waste which accumulates in the bottom of the fish tank, along with any uneaten food. Both solid fish wastes and uneaten food should be removed by the pump and deposited on the gravel of the hydroponic.

The fish are fed each day from a commercial food, which supplies all the nutrients and minerals the fish needs. This should include all the minerals required by the plants.

Gravel Microfilter

Plant roots cannot use fish wastes until microbes convert it to an available form. Some liquid fish wastes can be used by the plants, but most solid wastes require microbes.

The Microbes:

Microbes are organisms that can ordinarily be seen only under a microscope, such as algae, bacteria and fungus.

The minerals left over from the fish waste and uneaten food should supply the necessary minerals for the plants. However, the solid wastes are not in the correct form to be "available" for the plants.

The wastes have to be broken down by microbes before the plants can take them up. So the gravel is home to microbes that can use the solid waste as food. As the microbes use the solid wastes, some of them produce the forms of mineral necessary for the plants.

There are at least three distinct microhabitats in the gravel:

1) The top of the gravel is exposed to light, and so microbes in this area such as algae, are photosynthetic.

2) The mid layers of gravel in the hydroponic are not exposed to light, so the microbes in this area must get their food from other materials, such as fish tank solid wastes.

3) Below the drain in the hydroponic tank, the bottom gravel is kept in water so the microbes in this area must survive with very little oxygen and no light.

Water being used to raise fish can make an excellent hydroponic nutrient water. The wastes from the fish include liquid wastes and solid wastes. Both can be used in hydroponics.

The liquid wastes include ammonia (NH3) which is very good for green growth of plants such as leaves but poor for producing blooms and fruit.

The solid wastes from the fish have to be broken down by microbes before they turn into nitrates the plants can use. The microbes work best in a porous media such as grow rock but pea gravel is also commonly used.

The microbes that break down the solids should grow in the area of gravel between the surface and standing water. When grow rock is used they have a larger surface area to grow into, and so there tends to be a larger population in grow rock media than in pea gravel.

The microbes, once established, will use the fish solid waste as food, to both survive and reproduce. They turn the fish waste into carbon dioxide, water, heat and new bacteria cells.

There are bacteria that turn the waste into nitrites and nitrates for plant growth. If they are not present then the bacteria will break down the waste but not produce enough nitrate for plant growth.

One important point in fish solid waste is that the solid wastes have to be removed from the fish tank and transported to the hydroponics this can be accomplished by a pump that is strong enough to handle the waste, or a gravity feed system that collects waste at the bottom of the fish tank and moves it into the hydroponics.

A fish tank hydroponic can serve three basic functions:

1) Provide fish to eat

2) provide nutrient water for hydroponics

3) Provide a heat sink to store heat from the sun.

There is a balance between fish and plants that is required in a fishwater system, especially when it relies on enough plants to clear the fishwater.

Revised: 1 May 2021
Copyright © 2021 Institute of Simplified Hydroponics