algae in tank and number of fish

Question
You've been so helpful in the past, I chose to direct my question to you.  I keep getting brown algae on my decorations and plastic plants.  I've read that the cause is too many phosphates in my water.  Do I need to buy a phosphate test kit and buffer?  I change and vacuum 20% of water every week-two weeks.  I have two small Plecos but it seems to keep getting away from them.  I have taken out ornaments and scrubbed them and changed some plants I couldn't get clean.  I also changed the filter once after a thorough cleaning.  (it stunk)  Also, another expert told me I have too many fish in my 20L.  Could you confirm?  I have nine: a female Betta, two small Plecos, two Corycats, three Platys and a swordtail. Thanks again for all the help.  By the way, the tank was started in October.  It is, of course, cycled.  Thanks again for your help.  I always trust you.

Answer
Tricia,

 he algae problem can be a couple of things. How long is he light on? If it is on more than 12 hours a day, that could grow algae. Yes it could be a phosphate problem and I would test for it. It could also be because your nitrates can be elevated. As for the over crowding yes you have too many fish. A lot of people say it is a gallon of water to every inch of fish. This measurement is for an adult fish not a youngster. If you are going to keep all the fish then you will want to do water changes weekly and test the water daily. If the ammonia or nitrites are elevated then do a 25 % water change. Hope this helps. If you have any more questions, feel free to ask.