Greg Tong has provided the following excellent discussion of plants for goldfish aquariums. Be warned that, depending on the strength and enthusiasm of your goldfish, keeping plants may ultimately prove impossible. One clever trick is to mix plastic plants with real plants; the real plants help conceal the fact you have fake plants, while the plastic plants can endure the proclivities of your fish (and sometimes even protect the live plants). Just an idea. In reality, I just grow algae in my tanks!
Keeping aquatic plants with fish has more than aesthetic benefits and there are several kinds of aquatic plants that can survive in goldfish tanks. Unfortunately, most commonly available plants will not thrive in goldfish tank conditions because they are tropical plants.
To raise aquatic plants in a goldfish tank, you need to provide conditions that are good for the plants and the goldfish. Sometimes the two are very different. Thriving plants add beauty to a tank and can also help clean the water. Aquatic plants feed on fish wastes -- ammonia and nitrates -- removing them from the tank.
There is a risk, however, when plants don't thrive. Rotting leaves (and roots, which aren't visible) can pollute a tank quickly. Also, the plants have to contend with the goldfish's natural inclination to nibble and bulldoze anything in its way.
There is also a risk from infections brought in by new plants. For safety's sake, and because goldfish aren't known for their strong immune systems, disinfect your plants before putting them with your fish.
So what plants are safest for a goldfish tank? A short list follows. They all share the ability to resist goldfish nudges either with strong roots or by floating; and they all do well in cooler water.
Keep in mind that the suggested plants aren't likely to thrive like the ones we see in those glorious pictures in fish magazines. Under good, not even excellent, conditions, aquatic plants can grow an inch or a leaf a week, often more. In a goldfish tank, they would do well to hold their own.
For more detailed information about raising aquatic plants, visit the Krib plants section.
First, select plants that are suitable. Usually, these are plants that can hold their own in goldfish conditions, with strong root systems and temperate water requirements, versus tropical requirements.
Second, give your plants more light. The usual one- or two-lamp setups usually sold with tanks are not sufficient for plants. Consider adding one more lamp. The spectrum is not as important as the overall amount of light (lumens).
But don't keep the lights on more than 11 hours a day or overdo the lighting. More light also encourages algae growth when what you want to do is stimulate vascular plant growth so they win the competition against algae.
In addition, keep direct sunlight off the tank. Direct sunlight rapidly stimulates algae and can cause wide temperature fluctuations that stress the goldfish.
Third, fertilize lightly with trace elements but always avoid anything containing phosphates. Phosphates give algae an amazing boost. Your fish will provide an adequate amount of "macro" nutrients for the plants through their wastes. All you need to supply are "micro" nutrients.
First of all, algae is a good thing. Algae indicates healthy conditions in a tank. However, algae becomes a bad thing when it runs wild.
If your algae is taking over, scrape off as much as you can before a water change. Use the water change to siphon out floating bits. Many folks let algae grow on all but the viewing side of their goldfish tanks.
Then, there are only three practical things left to do. I call them:
(RWT: for my discussion on algae click here)
To be extra safe, disinfect new plants before adding them to an established tank. There are different ways to eliminate snails, which carry pathogens, destroy algae, and remove harmful bacteria.
I suggest a maximum two-minute dip in a solution of 1 part bleach and 19 parts water. This should not harm most of the plants suitable for a goldfish tank. For tender plants, dilute the solution or remove from the solution earlier. Of course, you will want to rinse the plants very carefully afterward, and use a dechlorinator if you'd like.
A few additional comments on the preparation of plants for the goldfish aquarium: depending on where you get the plants, they may have insect larvae, snails, and leeches and other parasites on them. They may also have been exposed to bacterial and viral fish diseases. The bleach treatment Greg describes above should help address some of the disease concerns. However, it will not even slow down things like snails and leeches. Also, tender plants like anacharis do not, in my experience, survive the rigors of a bleach treatment well.
One option is to quarantine the plants. A two week quarantine should eliminate the risk of introducing a parasite like ICH into your tank. A 48 hour dip in a solution of 1 tablespoon alum powder (from a pharmacy) per gallon of water will kill snails and their egg masses. Leeches and some insects can be killed with an organophosphate pesticide, like "Life Bearer" from Aquarium Pharmaceuticals (a several day soak at about 4X the recommended tank dosage). Air breathing insect larvae can be killed by doing the previous treatment in a zip lock bag with all the air removed.
Many people do none of these things and never have any problems ... it is up to you. At a minimum, give the plants a good rinse under a running tap!