Remember that any algae needs two nutrients, without which it cannot live/reproduce.

Nitrate and Phosphate. I suspect you might have a phosphate problem. Although all the other suggestions above would help alleviate the symptoms ie. green water, it does little to fix the cause.

Phosphates are introduced mostly by feeding, tap water and some cheap GAC (granular activated carbon).

I suggest the following:

1) If you have plants, cut your lighting in half. If you don't, switch your lights off for two weeks.

2) Change your GAC weekly. rinse it thoroughly with R.O. (Reverse Osmosis) water before adding it. You can get R.O. water at most good LFS's. While changing it weekly, two tablespoons is enough for 100L (per week). Using more could introduce more phosphates.

3) Cut your feeding in half for two weeks. No matter how little you feed currently. The fish will be ok

4) Most importantly, get a good quality granular phosphate remover to add to your GAC. Follow the manufacturers instructions.

5) Don't use any chemicals/fertilizers for at least 4 weeks.