It may just be a result of the age of the tank. An animal that eats it won't fix the problem. You have too much nitrate and phosphate, even if you can't measure it. Things you can do to help involve fixing the underlying problem:
Decrease feeding
Larger water changes
Use only RODI water
Decrease light
You can buy a GFO reactor to reduce phosphates
Macro algae either in a refugium or in tank.
Don't change too much at first. Make a one change and wait a couple weeks to see if it made a difference before you make the next change.
If you don't have them already, buy test kits for nitrate and phosphate so you can monitor the changes you make. If you write down the test results in a notebook you will be able to see what happens over time. The first entry should be before you do anything.
