Goldfish Tank

Question
Hi, Im currently having problems with a freshwater fish tank ive had for three months. It has three small goldfish in it, and is 20L. Lately, (in the past week) ive noticed that the water is becoming slightly cloudy, but this has never been a problem for me (in this tank and my previous tank). I change about 10-20% of the water every two-two and a half weeks and use a gravel cleaner but it doesnt seem to have cleared the water. My filter seems to be working fine, and is the under-gravel type. Ive already checked the ammount of gravel in my tank and have it so that it covers the filter but not by much (as per the instructions). Im thinking of getting another species in the tank; would you be able to recommend any? Any advice that could be given would be so much help! Thanks :)

Answer
Hello,
Well water cloudiness is caused by a few things: Algae growth, lack of oxygen, waste in the water. If there's algae in your tank you can get an aquatic Mystery Snail (Shell Colors: Gold, Dark Blue, Whitish-Ivory) which will eat the algae and are easy to take care of. Or you can have other bottom feeders such as:Corydoras catfish, Plecostamus, Loaches, and Ghost Shrimps.
You can also just get algae control and kill all the algae.
If you want more oxygen in your tank you can get an air pump and some airstones. If you want your tank to look extra fancy, you can even get a small/big aquatic plant.
Don't over feed or your fish will feel sick and it will cloud the water.
You can buy theses species and items @ your local petshop suck as Petsmart/Petco.
If the water is still cloudy, try changing the water!
Hope this helps and happy fish-keeping! :D
~Sam