i have a heavily planted 60 litre tank (planted with mexican oakleaf plant so many that there is no floor space left). 60cm tank and has a 13w tube lighting of roughly the same length. probably on like 10-12 hours a day. lately the tank is getting hammered with green algae. slimy stuff that is covering all the plant leafs and gravel, and not so much on the glass. i have to keep cleaning all the leafs with my fingers to stop them from rotting when they get covered in the algae. like a slimy skin comes off the leafs when remove it with fingers. any idea what might be wrong?