I have had my fish tank for 2 years now and it has only ever needed fully cleaning out once a month. Why all of a sudden is it going green after only 1-2 weeks of being cleaned out? The green that is appearing in the tank is not the normal dark green algae that naturally occurs but a more mid-green colour. Nothing has changed with regards to the tank apart from the addition of a small crab but I can not see him being the problem! Any thoughts please let me know.