If the tank has been set up for 5 months, than it has been cycled, cycle is the first 2 week process of building good bacteria and eliminating toxins, etc.
You would find (if tested) that your levels of nitrates, ammonia, etc is not at an optimal rate.
I could not really tell you what though, however, it sounds as though that they may all be out?..
Do not over clean the tank or filter, keep in mind that in nature their is algae, etc, that plays a key role in the enviroment!
Do you have any "Active Carbon, Ammonia Remover, Bio Noodles, etc"? That could be an issue?..
ALWAYS RINSE FILTER MEDIUM IN SYPHONED TANK WATER.
Try something like "Aquarium Life Liquid (advanced GEO-Liquid formula)". Top product, been using it on my tanks for a very long time.
I have a 55 Gal (all natural) cichlid/plant setup with bottom feeders and some community fish. Have not yet had any problems, i too have different kinds of algae growth in places that i have encouraged.
Hope this helps, good luck.