There is a fish shop on the Grand Market every Wednesday and Thurday, they have pumps, filters, etc. I think you should get a white bottle saying "Monitor" on it, it's a blue liquid to clean/clear the water. There are directions in Thai on the bottle...
Do you have a pump circulating the water, or at least an air pump with a filter?
If you have a water pump with a built in filter (usually a small area of the aquarium with extra glass walls and an overflow), you should add the "bio balls" (small plastic balls a little smaller than a golf ball) they help growing and keeping the cleaning bacteria.
How big is the aquarium?
I have 4 aquariums here, and the ones giving me trouble sometimes are small... the big one is doing the best, but it is also the only one with a water pump and built in filter...
Is the water really green, or just dark (brown)? The only time I had dark (brown) water coming out of the tab was when the water tank was nearly empty and the new water flowing in was stirring up the sediments from the bottom of the tank.
I have some test strips here, if you want I can give you a couple to find out about the water quality etc. it's one of those 5-in-1 tests, I had to ask friends to bring them over, I don't think you can get those here.
I had a problem with high nitrite count, which kept killing my plants. I found a cool website for this problem:
http://www.aquabotanic.com/DIYCO2.htm
There are some helpful articles on this website:
http://www.netpets.com/fish/fishref.html it is terribly formatted, active links turns white as the background, hehe. Take a look here about algea:
http://www.netpets.com/fish/reference/f ... gaepa.html - it might just help putting in some real plants...