Why is my fish tank water turning green after a few weeks, what to do?

Weekly to biweekly 25 % water change can keep the water healthy. Also, monitor the temperature and water chemistry and adjust as needed. You many consider wiping down the sides often to prevent algae from Growing on the glass. Best of luck and thank you for using Pet Coach.

Check the water chemistry using a home test kit or submit a water sample for testing. The local Petco may do this for free or for a small fee. Aim for an absence (0 ppm) of ammonia and nitrites with nitrates at 20ppm or less. Perform partial water changes to stabilize chemistry and/or to improve water quality. Rinse the filter media in tank water, not tap water, and replace it. Use only conditioned water that is temperature matched to prevent stress or shock. To limit algae growth, maintain pristine water quality via filtration and regular water changes. If possible, turn up filter flow to keep water moving. If this stresses the fish though, do not do it. Do not overfeed and remove any uneaten food promptly. Limit lighting to 8 hours daily and closely monitor water temperature as well. All these can promote algae overgrowth.

Goldfish are surface breathers so seeing them up at the top is not unusual. They are also very dirty fish, so I recommend getting a small pump for aeration and doing a partial water change. There is usually a period of cloudy water after establishing a new fish tank until the bacterial balance is established and stabilized in the tank. I also recommend getting a siphon--type tank cleaner so you can use it to clean the gravel as you remove some of the water. It acts as a vacuum to pull out all the fish poo that settles to the bottom, and the water you remove from the tank is a great thing to water any plants you have with, as it also fertilizes them. Remove no more than half of the tank's water and replace it with clean fresh water that has had the correct amount of water conditioner for the volume you're adding.

Brown algae overgrowth can be due to high nitrates, too much light or overfeeding. Use a liquid home test kit or submit a water sample for testing. Most pet or fish stores will do this for free of for a small fee. If nitrates are over 40ppm, it can be decreased by performing partial water changes. Use only aged or conditioned water that is temperature matched to prevent stress or shock. You can also use a nitrate absorbing filter insert. The algae will need to be physically scrubbed from plants or rocks and can be scraped from glass. Rinse filter media using tank water NOT tap water. Limit light to 6 hours daily, 8 if you have live plants and do not over feed.

Too much light, whether natural or artificial (green/yellow LEDs can worsen the issue) can sometimes cause an increase in algae growth. Moving the light around a bit to see if it helps, changing to a different type of LED, or just cleaning the tank more regularly/adding in algae eaters may help.

The cone-shaped snails you are referring to are most likely Malaysian Trumpet Snails. A sustained population of Malaysian Trumpet Snails is supported by an abundant food source usually in the form of organic debris or sludge in the substrate. The green water algae can also be attributed to excess nutrients in your aquarium. If you haven't done so already, be in the practice of siphoning your substrate during every water change to remove trapped organic debris and prevent build up. While it maybe tempting to siphon your entire substrate bed, be sure to clean no more than 1/3 at a time to maintain a healthy population of beneficial bacteria that live in the substrate and make up a significant portion of your biological filter. Be sure to use a dechlorinator and a bacterial additive with each water change to ensure a healthy environment for your fish and biological filter. The population of Malaysian Trumpet Snails will diminish naturally over time as you reduce their food source. In addition to enhancing your maintenance regimen, consider the introduction of Assassin Snails for additional natural control. http://www.liveaquaria.com/product/prod_display.cfm?c=1075+3106&pcatid=3106

Algae is a constant in an aquarium. The smaller the tank the harder to keep it clean. Bleach will kill any fish you put in the tank after using it, and it will just come back anyway. Frequent partial water changes will help, but manually wiping the tank is the only way to keep it clean. An algae eater can help but usually not a good idea with a beta fish.