Aquarium keeping does not have to be a hobby that endangers the environment. There are steps you can take to ensure that your aquarium will not have a negative impact on natural ecosystems.
Firstly when you go to a pet store to buy fish or plants, ask the retailer where the specimens came from. If they say they do not know, do not buy anything. You will want to know whether the specimens were bred in captivity to sell specifically for aquariums or if they were captured from the wild. Always go for the plants and fish that were bred in captivity. The less that aquarists buy organisms that were captured in the wild, the less the demand will be for wild fish and plants, and the less they will be harvested from their natural ecosystems.
Some times aquarists are given an organism they can not keep, or they buy one too many, or they simply decide they want to get rid of a perfectly healthy, living plant or fish. Believe it or not, letting your unwanted goldfish loose in a pond is actually a bad idea. An organism that is released into the wild when it has only lived in captivity has little hope of survival. They do not understand their environment. They will not know what their natural food source is or how to hide from a predator. In the unlikelihood of survival, the organism becomes what is called an invasive species. These are organisms that are introduced to the environment and harm it. They degrade ecosystem functions. Invasive species also affect human day-to-day life. Because they upset ecosystems where they do not belong, whatever waters they are introduced to become unsafe to use for recreation. ‘Drinking water plants’ have to work harder to keep the water drinkable, which costs more money.
There are alternatives to releasing an unwanted fish or plant into the great unknowns of the wild. Contact the retailer you bought it from and ask them about returns. If they will not take the fish back they might have some ideas about how to get rid of it. Another option is to trade or give it to another aquarist. Plants are easier-just seal them in a plastic bag and throw them away with the rubbish.
The best and really only way you can take steps to setting up an environmentally friendly freshwater aquarium is to think carefully about what fish and plants you want before buying. It is easier to buy few organisms and add on to your collection than it is to take organisms away! Your collection should only consist of specimens that were bred in captivity specifically to be sold for home aquariums. Follow those two steps and you will have a very environmentally friendly freshwater aquarium.